Ellen first appears in the JGSDF base, when Isaac Westcott goes to investigate the Wizard who could handle the <White Licorice>. While Ellen remains silent during the trial, Westcott asks her to give Origami his business card. After that, she and Westcott are discussing how foolish the higher-ups of the JGSDF are. When he makes a joke about Origami being stronger than Ellen, she changes the subject and shows him a file which contains information of a girl that resembles a Spirit, Tohka Yatogami. After Westcott reaffirms that the JGSDF are fools, he decides to take the matter into his own hands, and asks Ellen, the strongest Wizard, to investigate the Spirit <Princess>.
When Shido and his classmates are on the plane to Arubi Island, Ellen (who claims to be a photographer), takes a picture of Tohka while she was making a ruckus. Ellen apologizes for her rudeness, and sends what she discovered to the DEM airship, <Arbatel>. On the airship, the captain, James A. Paddington, is discussing with Ellen about Tohka's identity, and she reminds him he shouldn't let his guard down, for this is a Level 1 alert. Ellen gives the order to turn off the radio wave transmission. When Paddington asks her about the young Wizard, Origami Tobiichi, they are interrupted by a sudden storm.
Much later, Ellen is still spying on Tohka, only to be ambushed by Ai, Mai and Mii, and ends up getting caught into a pillow fight, before she could give more orders to <Arbatel>. She ends up falling asleep next to her target. The next day, she is still observing Tohka and reporting to <Arbatel>. This time, she watches Tohka and Origami swimming in a straight line, and asks <Arbatel> to deploy a <Bandersnatch> to follow her. However, she is once again disturbed by Ai, Mai, and Mii, and even though she says that she has important things to do, the trio and some students grab her body and throw her inside a hole in the beach. The sculpture made under Ellen's face was a masochistic act between a Queen and her servant, who happened to be Hiroto Tonomachi. That was the first time in her life that she felt like spitting on someone else.
Some hours later, Ellen manages to get out of the hole (although she buries Tonomachi in it), and informs <Arbatel> that she'll use her favored method to capture Tohka tonight. Some time after that, Ellen is almost caught again by Mii, but she manages to escape in panic. She goes outside following Tohka and asks <Arbatel> to send one <Bandersnatch>, just in case Origami tries to do something.
While Shido and Tohka witness Kaguya and Yuzuru fighting to the death, they are suddenly surrounded by 10 <Bandersnatch> units, and Ellen confronts the two of them. Ellen taunts Tohka and they have a fight. However, Ellen quickly beats Tohka using <Pendragon> while expressing her disappointment over a lack of a proper fight. Ellen orders the <Bandersnacth> to capture Tohka and restrain Shido. This is when Shido first manifests <Sandalphon>. Ellen is surprised that a human can use an Angel, and asks Shido to come with her too. When the <Bandersnatch> try to restrain Shido, <Arbatel>'s Realizer gets hit, and the robots stop functioning. Shido and Tohka escape, but Ellen gives chase. However, she falls into a hole that's supposedly the work of the Ai, Mai, and Mii trio. Next, a <Bandersnatch> falls on top of her and she can't get out since she had already released her CR-Unit. Ellen stops moving while cursing her luck.
After that, she wakes up, and tries to communicate with <Arbatel>. However, the only reply comes from Westcott. He asks her if she was able to confirm that Tohka was a Spirit, to which she replies that she is. She also takes the chance to ask him if he thinks something like a human capable of holding the power of a Spirit exists.
Ellen is with Westcott in the imperial hotel. He is checking her report on what happened in Arubi Island. When Westcott seems happy that he lost, Elen expresses her discomfort. She informs him that a team led by Adeptus 3 is ready to enter the JGSDF. After that, Westcott remarks that there aren't any photos of Shido, to which Ellen replies that she thought only Tohka was important. When she says that she'll get a photo of him soon, Westcott stops her, saying that he wants to meet him, to which Ellen replies "Understood. Without fail." Westcott also informs her that he has prepared a big welcome to <Ratatoskr>.
After Miku brainwashes her audience, Ellen appears through a hole in the ceiling and tries to capture Tohka and Shido (who was crossdressing). Ellen is surprised (and annoyed) when Tohka throws Shido out through a hole in the wall. She is also surprised that there are this many Spirits together. She then remembers that her priority is Tohka, and they both fight resulting in Ellen defeating Tohka. Moments later, Shido watches Ellen flying away with an unconscious Tohka.
While Tohka is held captive in the DEM building, Ellen enters the isolation room where she's at, and asks her to calm down, further provoking Tohka. Ellen informs Tohka that Shido, as well as the other Spirits, will be joining her within the next few days. Since Tohka was about to break her handcuffs, Ellen activates her Territory, which crushes Tohka. After Ellen claims that her Territory is the strongest one among all the Wizards, she asks Tohka some questions.
Some time later, Tohka is still being questioned by Ellen. Tohka refuses to answer, but Ellen doesn't seem to care. At some point, Tohka asks Ellen why a cameraman is doing this. When Ellen replies that she was masquerading as one, Tohka doesn't really understand. At some point, Westcott wants to enter the room. At first, Ellen hesitates to let him in, but after saying that as long as she's there he'll be protected, she lets him in. Westcott tells Ellen about several torture methods to inverse Tohka, and when he asks her about the one closest to her, Ellen replies that it should be Shido Itsuka. When Tohka is once again about to break the handcuffs, Ellen uses her Territory again.
Later, when Mana is fighting Jessica on the DEM building, Ellen suddenly appears, and insults her by saying she's a rat who joined <Ratatoskr>. When Mana said that DEM messes with one's body, Ellen just said that she won't betray Westcott. At some point during the fight, Origami appears and saves Mana. When she heads towards the DEM building, Ellen stops her, and starts a fight with Origami. During the fight, due to Origami's strategy and <Ratatoskr> support, Ellen is wounded by Origami, and claims that she's the second person who's ever managed to hurt her. After this, Westcott contacts Ellen, and she goes to the DEM building.
Once there, she goes to the isolation room and stabs Shido, saying that she'll destroy any sword pointed at Westcott. Ellen asks if this was alright, and then she begins to threaten to kill Shido with her sword. Tohka tries to stop her, but seeing that she can't, she inverses. Ellen asks Westcott about the situation, and he says that the demon king has come. Westcott tells Ellen that this is their dream, and asks her to defeat Tohka. The Inverse Spirit and the Wizard engage into a fight. Even though Ellen provokes Tohka, the latter has the upper hand, and even resists Ellen's [Rhongomiant]. Tohka takes the chance to point her sword at Westcott, distracting Ellen and causing her to reopen her wound. Westcott asks her what does she think about Tohka now, and Ellen claims that she's more powerful, compared to what she was before. Even though Ellen claims that she can win, she says that her wound will prevent her from fighting at full capacity. To this, Westcott suggests they retreat, saying that today was already a great achievement. Then, they both leave, while Westcott claims to know nothing about Shido Itsuka.
At the DEM company building in Great Britain, the DEM board members are reviewing Westcott's actions in Japan and, displeased, call for a vote to remove him. When half the board members raise their hands, Ellen uses a laser blade to cut their arms off. Then, she scolds the board for saying that Westcott is doing things out of "fun and self-gratification". Westcott, however, tells Ellen that the board members were in their right to call for a vote.
After Westcott talked with Woodman, he tells Ellen that Murdoch was right, that it was bad to lose Adeptus 2 and Adeptus 3 during the battle, and he suggests to look for new candidates. Ellen has no idea who could be worthy, except for the comatose Artemisia Bell Ashcroft. Westcott interrupts her, and he mentions the young Wizard who managed to hurt her.
When Ellen and Westcott arrive at Tenguu city from Great Britain, Westcott suggests they build a residence in Japan, since the trip is tiring. Ellen, however, seems displeased, since Westcott left DEM defenseless against the board members. While he is not bothered by the fact that they could cause a revolution, Ellen says that he should place in her situation, since she would have to do the cleaning afterwards. Westcott changes the subject to Shido Itsuka, and Ellen shows him what she's found. Apparently, Shido is surrounded by 6 Spirits, and he is probably backed up by <Ratatoskr>. They decide to wait for Shido to deepen his bonds with the Spirits before attempting to inverse them, and Westcott tells Ellen about the Spirit <Witch>, who hasn't been reported as lost yet.
Some time later, Natsumi is thinking about other pranks to pull on Shido, but Ellen and other Wizards find her. They engage in a fight. After Natsumi uses <Haniel> several times on the Wizards, they end up becoming children. Ellen decides that she can't waste any more time and attacks Natsumi with her laser blade, causing her to release her powers, and the Wizards go back to normal. When Ellen is about to cut off Natsumi's limbs, Shido and the Spirits arrive to save Natsumi. Despite this, Ellen still has the upper hand, so Shido and the girls teleport with Natsumi to <Fraxinus>.
After the fight, several men try to ambush Origami, since her disciplinary punishment has been decided. At the same time, she receives a call from Ellen, who asks her to join DEM. Origami refuses at first, but Ellen claims that they won't hurt Shido in the meantime, and that she could have access to special Realizers and information on the Spirit that killed her parents. Origami accepts the offer, Ellen attacks the group of men, and she welcomes Origami into DEM Industries.
After Natsumi escapes from <Ratatoskr>, Shido looks for her in his house, only to find Ellen there, sitting on a sofa. When he tries to call Kotori, Ellen takes his phone, saying that the whole house is under her Territory. She also wants to ask Shido where Natsumi is. He refuses, and Ellen tells him that the peaceful days he has with the Spirits is only because DEM allows it. In exchange for Natsumi, Ellen will let him and the other Spirits live. Shido still refuses, and Ellen takes out her sword. When she threatens to cut his ear off, Shido's phone rings, and he takes the chance to run away, since Ellen is distracted. However, she redeploys her Territory once again. This time, she receives a call from headquarters, and leaves before saying that Shido is a lucky person.
While she's out on the street, she explains the situation to a subordinate. An artificial satellite is falling on Tenguu city, and it's aiming at Westcott. Apparently, the board directors are trying to seize power by killing Westcott. Ellen feels regret for not chopping off their heads instead of their arms. She gives the subordinate orders to evacuate and to retrieve any Sephira Crystal that may be left behind.
Once she arrives at the hotel where Westcott's at, he tells her that everything's fine, since <Ratatoskr> won't let Shido and the Spirits die. Ellen says that even though her Territory could keep the damages to a minimum, it's still too dangerous. Since she insists, Westcott sends Origami with the <Mordred> unit to take care of the satellite.
In the highest suite room of a Tengo city hotel, Ellen asks Westcott if it's alright to let Origami, who has become the new Adeptus 2, to move on her own. He answers that he doesn't mind, and since it's getting hard to keep the JGSDF at bay, they could use Sephira Crystals and <Mordred> combat data. Westcott adds that if Origami fights the Spirits, there's no doubt that <Ratatoskr> will disturb her. Thus, he asked the HQ to send the airship <Goetia>. When she hears this, Ellen asks if it's really okay, since she can't hold back when using <Goetia>.
A while later, when Kotori decides to use <Fraxinus> to rescue Tohka, Kaguya, Yuzuru and Miku, the ship is attacked by Goetia. Ellen communicates with Kotori, and after telling her that her age or looks don't affect her personal abilities, she voices two requests. First: everyone in <Fraxinus> has three minutes to evacuate the ship. Second: the escaped survivors will give Woodman a message from Ellen, saying that she's coming for him. Then, a huge battle ensues between both airships and, after <Ratatoskr> believed they had won, <Goetia> fires at <Fraxinus>, greatly damaging it.
After the fight and some other missions, Ellen returns to Westcott. While he congratulates her, she regrets the fact that <Goetia> got hit once, and claims that <Ratatoskr> has an outstanding crew. After this, they talk about Origami, who became a Spirit recently. While she was an amazing Wizard, they are still going to target her.
Once Inverse Origami appears, Shido and the others try to get to her, but they are attacked by Ellen, who claims that they can't hurt an Inverse type. However, Kaguya and Yuzuru manage to distract her, giving Shido enough time to reach Origami.
In the DEM Industry building, Westcott is talking about how he regrets the loss of <Devil>, to which Ellen apologizes, though it wasn't her fault. Westcott changes the subject to <Material A> in Neryl Island. When she hears that name, Ellen twitches her shoulders, since <Material A> is DEM's most prized possession. According to Westcott, it's time they make their own Inverse Spirit.
Soon afterwards, Ellen decides to go out to eat her favorite strawberry shortcake, but the cold weather annoys her. She remembers a subordinate who told her "Japan's winter is not a big deal". Then again, that subordinate was from Alaska. Suddenly, while thinking about adding strawberry shortcake to the company menu, she bumps into Shido. He tells her that he had feelings for her since their last meeting. And even though she is from DEM, she should go over to <Ratatoskr>. Ellen thinks this is a prank, and activates her Territory to crush Shido. However, he is unaffected by it. Then, she starts attacking Shido, but he dodges and blocks all of her attacks. After some more attacking, Shido lifts Ellen in a princess carry manner. Next, Shido decides to kiss Ellen, but ends up kissing her forehead since he doesn't want to force an unwilling girl. He goes away, and Ellen claims that she'll never forget this. With teary eyes, she returns to the company building, escaping from the public eye and from her chance to eat strawberry shortcake.
Back at the company building, she makes her way to Westcott's office, while thinking about what happened with Shido. Irritated, she hits the wall, but the pain makes her crouch down on the spot with teary eyes. Once with Westcott, he informs her that <Nightmare> attacked the transporter at Neryl Island, but <Material A> is safe, thanks to the new Adeptus 2. However, the reason Westcott called Ellen was because they had detected some unusual readings. When she hears this, Ellen's encounter with Shido made sense to her. When she's about to leave, she claims she'll bring the target's head, but Westcott asks for a living sample.
Later, while the Spirits try to save Shido, Ellen attacks the facility with 20 <Bandersnatch> units, but they are all destroyed by a mysterious light beam. When she sends the next wave of robots, they are beheaded by Mana. Ellen remarks that Westcott should have let her kill them "at that time". Nonetheless, both Wizards engage into a fight.
While still fighting, Shido escapes <Ratatoskr>'s facility, distracting Mana, and giving Ellen the chance to attack her. When he mentions the name Mio, Mana gets lost in thought, and Ellen prepares an attack. However, Shido uses his powers to block her attack, and Ellen says that she now understands why Westcott wants him so much. Shido releases more Spirit energy, and both Mana and Ellen are blown away. After that, Kotori contacts Mana, and she keeps Ellen distracted while the girls go save Shido.
During their fight, Mana provokes Ellen by naming some of her faults, but Ellen receives a call regarding the status of <Material A>. Ellen escapes, but tells Mana that she won't be so lucky next time.
In the DEM building, the pilots Knox and Burton are summoned by Westcott, regarding the loss of <Material A>. Since the Spirit escaped, and Ellen Mathers is with him from the beginning, they are sure she'll kill them. However, she gives Westcott a report on the matter. Westcott, however, reassures them, saying that it wasn't their fault, and that it wouldn't be good to lose such good pilots. Later, Nia reveals to Shido that she is a Spirit, and she mentions that she was captured by Ellen, who ambushed her and overwhelmed her. Several days later, Westcott, Ellen, and Artemisia are driving through Tengu city while speaking of Nia. While Ellen dislikes the fact that they let her go, she is reminded that they hadn't made any progress in inversing her. This time, however, it will be different. Westcott tells Ellen that he is counting on her and Artemisia to achieve their dearest wish.
When Nia inverses, Shido and the others try to help her, but they are attacked by Ellen. She tells them that she'll let them go this time, but she's soon attacked by the shadows coming from <Beelzebub>. Then, she engages into a fight with Tohka, Kaguya, Yuzuru and Miku. Later, once Artemisia gives Westcott Nia's Qlipha Crystal, Ellen goes to him, and reminds him that only one Demon King is not enough to achieve their dream. However, Westcott says that this was enough for today, and retreats together with Ellen and Artemisia.
Back at DEM's building, Westcott is investigating <Beelzebub>'s power. When Ellen asks him if there's something interesting, he touches her shoulder, and she learns that the world's history got rewritten by Shido to save Origami. Ellen also learned that Origami wounded her, and this caused Ellen to "create" the wound she got at that time. Despite <Beelzebub>'s powers, the Demon King is not yet complete. Ellen claims that she should have removed the Crystal herself, but Westcott is satisfied nonetheless, and reaffirms his expectations on Ellen and Artemisia.
A few days later, Ellen felt exhausted after having to climb over the numerous stairs in DEM industries’ Japan branch. With her Legs cramped and lungs exhausted, Ellen wails in agony over how the elevator is broken. Soon after, she is joined by Artemisia, who asks if she is okay. Rejecting the need for sympathy, Ellen tells Artemisia that there is no need for any assistance. Ellen complains that it is only because she went swimming earlier. However, Artemisia reminds Ellen that she also went swimming, and had done more laps than Ellen. While Artemisia swam gracefully, Ellen had to use a flotation device she had nickname Puriduen. Not wanting to keep Westcott waiting, Artemisia decides to carry Ellen for the remainder of the stairs, much to Ellen's embarrassment as the posture evokes bad memories from the incident with Shido. After arriving, Westcott informs them that <Beelzebub> has been receiving interference from an outside source, which he theorizes came from <Sister>'s survival. Despite this heavily restricts the Demon King's omniscience function, he still managed to decipher the information just in time. Thanks to this, he has found the location of a new Spirit. When Ellen questions the Spirit's whereabouts, Westcott responds by raising his index finger and pointing to the sky.
After the initial assault, Ellen is shocked to find out that the fleet sent to capture the new Spirit <Zodiac> has been completely annihilated. Asking if the Spirit is still at its previous coordinates, Ellen decides to venture out herself in order necessitate the capture. Seeing Ellen's resolve, Westcott assigns Ellen and Artemisia to apprehend <Zodiac>. Much later, during the new Fraxinus EX maiden voyage, they are soon intercepted by three DEM airships led by Ellen's <Goetia>.
Using the communication system, Ellen contacts the Fraxinus ship and warns them to surrender or else they will be brought down again before the might of her fleet. Hearing this surprises Kotori, who thought that Ellen should have no memories of the previous timeline. However, the Fraxinus commander quickly realizes that Westcott had probably used <Beelzebub> to obtain the information. Still, Kotori notices of Ellen's presence in space while Westcott had left to personally attack the Ratatoskr base. Despite clearly being affected by the taunt, Ellen denies becoming flustered as she states that Westcott wouldn't leave and attack Woodman without her. Resuming a sharp demeanor, Ellen states that five minutes should be enough to annihilate their ship.
Using <Goetia>'s flexible locomotive ability, Ellen is able to dodge the attack fired from Fraxinus' <Mystletainn> cannon. She admits that she is impressed by how Fraxinus had used its territory to copy <Goetia>'s agility, but still boasts by correcting herself that three minutes should be enough even with the improvements made to Fraxinus. Still, Ellen also mentally notes that she can't completely destroy the ship because of the Spirits onboard, and doing so might scatter the Sephira Crystals in space forever. She then boasts that challenging her is like trying to fly too close to the sun. The Fraxinus ship would soon have its wings clipped and crash down to the Earth like Icarus. However, Ellen is caught off guard by a large wave of energy that pierces through <Goetia>'s defensive hull. Shocked, she realizes that the energy beam was powered by energy from the Spirits. With the severe damage done to <Goetia>, Ellen is forced to retreat back down to Earth as Kotori mocks her.
Three hours after the semi-wrecked <Goetia> landed on the surface, Ellen arrives back at DEM headquarters. Her mind is filled blood filled hatred towards the Fraxinus for giving the first humilaiting loss in her life and exasperated wrath at Westcott for hiding the raid on the Ratatoskr base from her. In her rage, he dismisses her subordinates' concerns about her own injuries and asks for Westcott's location. Along the way, she is greeted by Artemisia, who states that she came from the weapon storage room as fast as she could. Ellen rebukes Artemisia attempts to check her injuries, asking if the second Adeptus only wished to mock her. Much to Ellen's frustration, she finds that Westcott was in the middle of a Realizer treatment to recover from his wounds. Despite this, the DEM director happily waves his severed limb in front of both Ellen and Artemisia. He admits that he was magnificently defeated by Ratatoskr. Nevertheless, the front half of his severed arm was successfully recovered, so he should still make a full recovery by tomorrow. After hearing the medical staff's concern over continuing his treatment, Westcott tells them that he will have to postpone their chat until the next day and they should go heal their own wounds in the meantime. Having exhausted her stamina, Ellen collapses just as a medic timidly asks if they could examine her injuries.