In the beginning, Natsumi had a one-sided hate relationship towards Shido, but now she mellowed out and has an affectionate feeling for him.
While their date was originally going smoothly, Natsumi turned hostile when she believed Shido discovered her true form, so hostile in fact, that she tried to ruin his reputation, erase him, and all of his friends in a twisted game. Later, after he saved her from Ellen and tried to convince her that her true form was already cute, she gradually opened up to him. She had shown remorse over what she put him through. However, when Shido remembered being nearly killed by being shot, frozen, eaten, incinerated, blown away, or attacked by a huge mob, he considered her actions to be mere childish pranks, surprising Nastumi about his experiences. Shido currently treats Natsumi as one of his friends.
At first, Natsumi simply used Yoshino's puppet Yoshinon as her disguise during her 'game' and it is later revealed that Natsumi was jealous of Yoshino because of her cute appearance.
Natsumi thought that Yoshino and the other spirits would hate her for her previous actions. However, when she was posing as Kotori and asked them about it, it turned out that they were all willing to forgive her and would like to know her better, much to Natsumi's shock and disbelieve.
After Natsumi's spirit mana was sealed by Shido, the two of them became good friends due to their similar personalities and the fact that they don't go to school and stay home together. Natsumi has become very protective of Yoshino, as seen when she uppercutted Shido when she believed he sexually harassed her. However, in turn Natsumi often hides behind Yoshino if she is scared of something, Miku for example. Natsumi is often touched by Yoshino's kindness and appears to view her as a goddess.
Initially, Natsumi viewed Miku as another pawn in her game, and took particular delight in terrifying her. However, Miku soon reveals that she holds no grudge on Natsumi for doing so, as Miku relates that she did similar violent actions when she still had full access to her powers.
After being sealed, Natsumi regards Miku as dangerous for her yuri personality. However, Miku still makes the extreme effort to try and befriend her, and often clings to Natsumi whenever she is excited. During Mana Research, the two of them went shopping together after Miku says that Shido and Yoshino might compliment her if she wore new clothes.
Out of all of the other Spirits' past, Natsumi's story in particular resonated the most within Nia's heart, especially Natsumi's words of wanting to teach Nia how beautiful friendship can be. After Nia is sealed, she is impressed by Natsumi's drawing skills, and approached Natsumi about the potential of working for her as an assistant. However, Natsumi remains hesitant about the offer, especially since Nia made it with her usual teasing manner.