After Shido's meeting with Woodman and Karen, Shido responded to the spacequake alarm and found Natsumi in an abandoned amusement park on the outskirts of town. However, Natsumi had used her powers to transform the surroundings into distorted gothic structures and cross gravestones. The conversation between the two started off well, with Shido responding to Natsumi requests for praise by complementing her on her appearance. But soon, the AST squad came and launched their attack. Natsumi easily turned their missiles into carrots, and the AST members into cute dogs, pandas, and rabbits.
However, a problem arose when a stray carrot missile exploded near where Natsumi and Shido were standing and threw dust into the air. This caused Natsumi to temporarily release her transformation power and caused Shido to cover his eye from the light emitted from Natsumi. When Shido was able see again, Natsumi got angry at Shido because she thought that he had seen her secret. She left the scene quickly after threatening Shido for knowing her secret.
The next day, Natsumi transformed herself into Shido and went on to harass everyone that was an acquaintance to Shido in order to ruin his reputation. Her various criminal acts include, groping Tohka, stealing the Yamai’s underwear, and doing nothing to Origami. When confronted by the real Shido at the rooftop, her disguise was seen through by both Tohka and Origami. Tohka used her high sense of smell, while Origami had kept track of Shido’s blinking rate through her stalker information.
Five days later, Natsumi sent Shido a letter issuing a challenge. The letter contained 12 photos, which Natsumi had impersonated as a person in one of the photos. The objective of the game was for Shido to find which person Natsumi transformed into. If he failed to guess the right one, then the real person would disappear. During the progress of her game, Natsumi watched in enjoyment as Shido guessed and failed each time. By the first day Yuzuru disappeared, the second day Ai and Yoshino, the third day Tohka, the fourth day Tama-chan Sensei and Tonomachi, and the fifth day Mai and Mii.
However, at the end, Shido figured out which one was Natsumi with a few helpful hints from the people that did not disappear yet. Origami raised the question if Natsumi could transform herself into something other than a Spirit or human. Also, Miku gave Shido her hairclip as a memento of her if she would disappear. Both suggestions caused Shido to realize that there were 13 suspects within the 12 photos: Natsumi had transformed herself into an inanimate object. At last Shido understand that Natsumi had transformed herself into Yoshino’s puppet Yoshinon. He reasoned that Yoshinon had knowledge of her time at Origami’s home, which was impossible since Yoshinon is a split personality of Yoshino, and should have no knowledge of events where Yoshino isn’t present.
Natsumi’s Angel released the captives as promised. However, at the same time, her own transformation is released and her normal form is revealed. She reacts violently, immediately activating her Angel and transforming everyone who saw her in her normal form into little kids, except Shido (The accumulated mana from previous Spirits in his body protected him), and left the scene afterward.
For the next several days, Natsumi uses her power to harass Shido and tries to turn him into a criminal. During these times, Natsumi speculates that Shido must have a large organization backing him up and to prove the point, is found out from her hidden spot as Shido closes in on her. Natsumi tries to run away but is stopped by the DEM wizard, Ellen and her DEM Teammates. At the beginning of the battle, Natsumi has the advantage by turning Ellen and all of her subordinates into children. However, Ellen takes the opportunity of Natsumi lowering her guard, to land a deceive wound on Natsumi, which reverts everyone she transformed back to normal. But before she could be captured by DEM, she is saved by Shido and his companions. Natsumi is confused as to why they would save her before she loses consciousness.
Natsumi wakes up to the place controlled by the organization which she assumed was backing up Shido. Due to her wound, she couldn’t use her power to escape. Here, Shido offers his own challenge to Natsumi. He and the rest would try to show that Natsumi could be beautiful without her transforming. If she still didn’t think so afterwards, they would let her go.
The beauty treatment begins with a massage salon by Miku, a hair salon by the Yamai Sisters, as well as a clothing and make up session with Shido (as Shiori). Despite being surprised by her new appearance, her pessimistic attitude remained and it was difficult for her to last a long conversation. Natsumi's conversations with both Tonomachi and Kannazuki ended both in failures due to her always interpreting compliments to be stealth insults or threats. Having recovered enough of her power, she transforms herself into Kotori in order to make her escape. However, when approached by the other Spirits, she is surprised that they did not insult her behind her back. Unable to bear with the strange interactions and events that happened around her, Natsumi makes her escape.
With the satellite impact arriving, Shido desperately searches through the city for Natsumi, trying to warn her to take shelter before the impact. She then watches Shido as he looked for her, and she felt something in her heart as she observed Shido trying his best to find her. She observes Shido not escaping when the spacequake alarm goes off in order to destroy a large falling object from the sky. But, when Shido is in danger and no one can save him, Natsumi appears against her will and saves him. Knowing that the satellite had explosives attached to it, Natsumi transforms the satellite into a mascot pig with cartoonish explosives. Then, she uses <Kaleidoscope> to transform her <Haniel> into a copy of <Sandalphon>. With the combined power of three Sandalphons, Natsumi, Shido, and Tohka, destroy the falling object and save the town.
After the battle, Natsumi cries as the group gave her unexpected praises for her actions. As they headed back to <Ratatoskr>, Natsumi pulls Shido to a spot where the other girls couldn’t see and kisses Shido. She did this knowing full well that it would seal her power, as she acquired this information before she escaped. The rest of the girls appear and scold Shido for making Natsumi cry.
As Shido wakes up from his sleep, he is surprised to find Natsumi, in her adult form, sitting next to him on his bed. Natsumi greets the startled Shido, who has quickly backed away from her. Shido is surprised to see Natsumi in this form, but Kotori enters the room and beats him up before he could figure out what's going on.
Kotori explains to Shido that while Natsumi's powers have been sealed successfully, her mental strength is extremely weak and her emotions are quite unstable. As a result, her powers manifest much more easily than Tohka and the other Spirits. Kotori tries to lecture Natsumi but they end up insulting each other about their bust sizes.
Kotori gets caught up in the moment and remarks that Natsumi only has a nice body thanks to her transformation and that her real body is shorter and smaller than Kotori's. The remark apparently upsets Natsumi so much that she starts crying. Kotori is forced to eat her own words and admits that she is more underdeveloped than Natsumi, only to realize that Natsumi has tricked her. Natsumi runs away laughing as Kotori chases after her.
Later, at the Spirit mansion next to the Itsuka household, Natsumi has changed back to her real form and is sulking alone. She had wanted to thank Shido by waking him up in the morning, but she got so nervous that she transformed into her adult form. As Natsumi spirals further down in her self-loathing, Yoshino pays her a visit. The two decide to have tea and sweets together. Yoshino offers to show Natsumi around in the city, which prompts Natsumi to ask the reason for why she's doing this. Yoshino says that she's happy that Natsumi is here and the two of them can talk to each other, because normally everyone else is at school around this time. Yoshino blushes and goes on to say that she and Natsumi are friends. Natsumi gets flustered too and offhandedly remarks that she'd marry Yoshino. Both of them become embarrassed, but Natsumi quickly changes the topic and off to seeing the city they go.
Back in the ruins, Shido is still desperately trying to free himself. To his surprise, Origami suddenly comes back. Shido knows that Origami is not the kind of person who would return without finishing what she has set out to accomplish. Fearing the worst, Shido asks Origami why she has come back, but she remains silent. Shido's trembling voice seems to have triggered a reaction from Origami. It's a reaction which Shido has never seen from her before. Origami smiled. Shido becomes unsettled and asks her why she's smiling. Origami doesn't answer him; instead, her smile gradually turns into loud laughter. Shido is completely baffled by the situation; Yoshino entering the room does not make it much better. Shido yells at Yoshino to run away because Origami is definitely not her normal self, but it turns out that the laughing Origami is actually Natsumi in disguise.
Apparently the two of them saw Origami on the street and backtracked their way to where Shido was imprisoned. Shido tries to contact Kotori but his call won't go through even on the Fraxinus' special line. Thinking that something must have happened to the Fraxinus, Shido asks Yoshino and Natsumi to guide him out of this place. Later, Natsumi is among the Spirits who make a commotion that prevents Shido from resealing Tohka.
Afterwards, they all return back to school, only to be immediately sent to a hospital after seeing their wounds. In the hospital, both Natsumi and Yoshino collapsed into sleep from exhaustion. However, the two woke up right as Shido was reaching for the phone that was vibrating in Yoshino's pocket. Misunderstanding the situation, Natsumi lands an uppercut on Shido. However, the peace is broken by Origami, who had inversed after returning from the past. Agreeing to help Shido reach Origami, Natsumi starts to transform the rubble into fluffy cotton.
In the new timeline, Kotori and Yoshino note the drastic drop in Shido's mood. Yoshinon wonders aloud why Shido is so listless. Natsumi says it's obviously got something to do with women, which alerts both Kotori and Yoshino. Natsumi concludes that high school boys' problems boil down to women but then goes on vividly describing a series of bullying incidents in schools. The girls decide to cheer up Shido by appealing to him using their feminine charms. Natsumi says she can still use some of her transformation powers. Kotori warns that if they were changed into little kids again it would only increase Shido's load as he turns into his male nurse mode. Natsumi says that this time it will be the opposite.
While preparing the dinner, Shido almost cuts his finger as he's distracted by thoughts about Origami. Kotori and Yoshino come to help, but Shido drops the knife when he turns around to see his sister and Yoshino, all grown up (to Shido's age) and dressed in suggesting clothing
The two girls grab Shido's arms and the usual shenanigans ensue. Shido realizes it's Natsumi's doing and yells at her. Natsumi comes out of hiding from behind the sofa. Despite also having grown, she's dressed in plain maid dress instead of Kotori and Yoshino's revealing clothing. Kotori gets in a bit of an argument with Natsumi about how they have all agreed to wear swimsuits. Kotori tries to take off Natsumi's maid dress and the two end up running around in the living room. Shido attempts to put a stop to the childishness. However, everyone ends up falling and crashing due to gravitational acceleration and Shido is treated to a spectacular view of the girl’s clothing starting to fall apart. The girls apologize to Shido and he thanks them for worrying about him and tells them that he's alright.
During the battle with Inverse Origami, Natsumi is among the sealed Spirits offering her aid. As Miku plays Rondo to push Origami down, while Natsumi, in her adult form, uses her <Kaleidoscope> to create an imitation of <Army Breaker Diva> and plays March to give Shido more strength. Miku asks Natsumi to pay her copyright royalties later, since an idol’s songs are serious business.
Natsumi is present at the gathering of the sealed Spirits in one of Ratatoskr’s underground bases. In her usual pessimistic attitude, she questions both Shido’s condition and tells everyone that they should be prepared for the worst. The Yamai Sisters quickly scold Nastumi for her negative thinking, telling her that Tohka would end up believing her words. As Origami begins to wonder about the nature of Shido's powers and Fraxinus’s motives for helping the Spirits, causing Natsumi asks her if she thinks Ratatoskr is hiding something from them. In response to Natsumi's claim, Origami states that she is grateful to Ratatoskr, but there is too much mysteries surrounding the organization and its motives for helping the Spirits. Much later, Reine and Kotori arrive and explain the situation with Shido, and how they each need to re-kiss him in order to restore the pathway that circulates their powers between them and Shido. However, much to everyone's surprise, Shido has already disappeared from his hospital bed. After finally finding Shido, Natsumi, like the other sealed spirits, is shocked by Shido’s new flirty attitude and how he wants everyone to make him fall in love.
As everyone else is going on their date with Shido, Natsumi tries hiding in the corner, worried that everything will end up failing because of her. Shido, quickly finds her and teases her by saying that he won't go on to the other Spirits until Natsumi makes his heart race. Seeing this Fraxinus begins the preparations for Natsumi's date with Shido. They give the command for her to try to seduce him in her Adult form. Reine quickly objects to her actions, telling Natsumi that using her powers is too dangerous with the pathway still blocked. However, she runs out of Spirit Power before she could finish her response. Nevertheless, the sight of her clothes fall off after post-transformation was enough to capture Shido.
During the final battle against Berserk Shido, Natsumi is the only one who doesn't equip her Limited Astral Dress, since she had already used up all of her Spirit Power in Shido’s capture. Instead, she follows Yoshino’s lead on top of <Zadkiel>. However, before she could kiss Shido, she goes on another one of her self-loathing induced rants. This gives Shido enough time to break out of Zadkiel’s ice restraints. Luckily, Miku intervenes and restrains Shido with her <Gabriel>. After both girls kissed Shido, Miku remarks the lucky opportunity in having a direct kiss with Darling and an indirect kiss with Natsumi. While rejecting Miku’s claim, she blushes at the realization that she had also just had an indirect kiss with Yoshino.
After the conclusion of the battle, Natsumi is one of the Spirits who walks in on the kiss between Shido and Tohka, commenting on how gross it was for them to kiss right after waking up.
After <Ratatoskr>‘s second failure in capturing Nia, Kotori reluctantly decides to ask the other Spirits for their help. After hearing that Nia could only love 2D, Natsumi suggests that she uses her <Haniel> to turn Shido into a book. Shido immediately protests to this action, which ended up making her depressed, especially after she felt that Shido’s attention increased after Origami proposed something similar. However, after it was decided that they would create a doujin about Shido, each of the Spirits drew their own sketches to see who the main artist would be. Natsumi’s illustrations are comparable to that of a pro, due to her talent at mimicking other people. As a result, it was decided that she would be the main artist. While she was initially shocked at getting the role, Shido’s pleas convince her to take on the job.
During the work process, Natsumi has the most exhausting work load, being in charge of drawing both the name and the cover illustration. Despite everyone’s insistence, Natsumi refuses to take any breaks. She states that she was saved by the kindness of Shido and the others; as such, she wants Nia to experience the same feeling. Not knowing if she could be helpful later on, she decides to give her all in what she can do now; the drawing. After finally finishing the illustration, Natsumi collapses in exhaustion. Shido takes Natsumi to her room for a well-deserved nap, and also to keep her out of Miku’s clutches. During the sales battle, Natsumi also helps out as one of the sales girls. When Miku announces that she had used her status as an idol in social media to attract more customers, Kotori gives the command that Natsumi needs to hug Miku to re-energize her whenever the idol was feeling tired, something which Natsumi protested against.
Afterwards, when Nia uses <Rasiel> in order to find out how Shido and the others managed to complete the doujin in only two days. It was then that Nia heard Natsumi’s words about wanting to teach Nia about the beauty of friendship. Subconsciously, due to her inner desire of wanting to be saved, <Rasiel> shows Nia each of the Spirit’s past, starting with Natsumi, and how they all opened up their hearts to Shido.
However, the situation took a turn for the worst, after Westcott’s actions causes Nia transform into her inverse form. During the ensuing battle, Natsumi used her <Haniel> to transform <Beelzebub>’s scattered pages into leaves. However, she was immobilized after Nia uses her future describing power to redirect <Haniel>’s light and transforms Natsumi into a mascot character. Like everyone else, she was shocked when Westcott manages to steal Nia’s Demon King. Immediately, afterwards, she uses her <Haniel> to transform Nia’s wounds into a body without any injuries. However, she wasn’t able to heal the damage done, and Nia’s condition continues to worsen until Shido was able to seal her and uses the flow of Reiryoku between him and the other Spirits to help Nia recover.
Later, Natsumi is present as Shido took everyone outside to see the first sunrise of the New Year. Much like everyone else, she is shocked when Nia states that every Spirit use to be human.
Three days later, the Natsumi joins Shido and the others Spirits in visiting the shrine for the first time in the New Year. While Shido's group was praying at the shrine, Natsumi is with Nia, drawing decorative illustrations on the wishing plaques. After being seen by Shido, Natsumi hides her drawing and claims that she only drew because Nia let her, not because she wanted to do so. In response, Nia teases her for being reclusive and says that moments ago they were talking about plot advancements in their manga. While denying this, Natsumi asks her what she meant by plot advancements. Nia remarks that Natsumi has a great future and even wants her to be her own personal assistant. She offers to pay Natsumi a salary and introduce her to an editor. As sweat dripped from Natsumi's cheeks, Nia makes a joke comparing her to a nut, which causes her to get angry at Shido for laughing. Extending her previous offer, Nia states that she wants to hire both Natsumi and Shido, as she could use Natsumi for reference in 18+ material, greatly shocking and disturbing the copycat Spirit. Hearing Natsumi's name being used, Miku rushes in and demands to be included as well.
After the initial failure to convince Mukuro, Natsumi and the others suddenly crash out of their hiding spot, revealing that they had been eavesdropping on the entire conversation. Everyone soon agrees to once again lend their power to help Shido. However, the question is raised on how they are going to approach Mukuro when she is in space. Kotori quickly remembers something and decides to take everyone into a secret Ratatoskr base where the new Fraxinus EX was being built. Natsumi and the others are then marveled by the presence of the Fraxinus EX's AI, MARIA, which can now communicate directly to the crew. Then, MARIA informs everyone that Ratatoskr's founder, Elliot Woodman, would like to have a word with them.
After Woodman explains his reasons for leaving DEM and creating Ratatoskr, he introduces everyone to Karen, who had also left DEM with him. Hearing that Karen is Ellen's younger sister, Miku starts breathing heavily while commenting on the sisterly development. While doing so, she hugs Natsumi tightly, causing Natsumi to struggle in order to break free. However, the peaceful conversation is interrupted by DEM, who had found their location from <Beelzebub> and has begun their attack. As they make there way for the Fraxinus EX, Natsumi and the others accidentally run into Westcott in the process. Westcott uses <Beelzebub>'s ability <Ashufiriya> to transport everyone into a world of fantasy. Despite their attempts to resist, they are all swallowed into the respective books that emerge beneath their feet.
In the fantasy world, Natsumi reunites with Shido and the others just as they are rejected entry to the king's ball. Much to everyone's shock Natsumi is wearing a decorative dress while in her adult form. Kotori asks her if she is able to use <Haniel> to change her appearance. However, Natsumi responds that she met a magician that transformed her appearance. Using her newly found beauty, she tries to seduce the guard into letting them in. However, Natsumi transforms back into her normal appearance as the clock strikes 12, causing her to hide behind Yoshino after the angered guard accuses her of being a witch. Shido notes that Natsumi's story must be based on Cinderella, and that the magic of the story always ends at midnight. Eventually, they decide to use Kotori's matches to create illusions of beautiful dresses, that allowed them to sneak into the party.
There, they found Miku as the mermaid entertainment, and Origami as the king of the castle. After finding everyone, they start brainstorming again for ideas of how to escape from the fantasy world. However, at that moment, the castle is suddenly invaded by the villains from each of their respective stories. For Natsumi, Cinderella's stepmother and stepsisters appear to taunt her for having the gall to show up at such a fancy ball. Since they couldn't use their angels in the fantasy world, Natsumi and the others are helpless against the ensuing onslaught. Despite Natsumi's attempts to impede the him, the wolf began his preparations to eat Yoshino as the first victim, Luckily, they are saved by the appearance of a second Shido, who Natsumi recognizes the doppelganger as the main character of the manga that everyone wrote to impress Nia. After the other Shido manages to defeat all of the villains, he opens a gateway back to their world with his <Sandalphon>.
After returning to their world, Natsumi and the others board the new Fraxinus EX and launches off into space in order to begin the mission to save Mukuro. After seeing Mukuro, she comments that she looks scary, and questions if they should go together. However, Kotori relents that they should only interfere if Shido is in dire need of help. At that moment, while Shido converses with Mukuro, Natsumi and the other's attention is diverted by the four DEM warships approaching them.
As Kotori and the rest of the Fraxinus crew prepare for combat against Ellen's <Goetia>, the Fraxinus commander finds that Natsumi is missing and assumes that she had run away scared. However, Natsumi had actually tagged along with Tohka and Origami in order to help Shido against Mukuro. Transforming herself into a scrap piece of metal, she pushes Shido out of the way from a potentially devastating attack initiated by Mukuro. Afterwards, Natsumi coordinates her movements to Shido's plan by disguising herself as him in order to provide a distraction for the real one to be able to unlock Mukuro's heart.