Nitori Shuuichi is the protagonist of the series. Nitori lives much of her life assigned male at birth (AMAB), but throughout the series displays an affinity for presenting as female. The manga follows her life from fifth grade (at the start) to the manga's end, when she leaves off to college. She's voiced by Hatakeyama Kousuke in the anime.
Nitori is described as a quiet, docile child. Nitori has been shown to enjoy baking. Nitori is called "Shuu" by her family members and "Nitorin" by Makoto.
Throughout the series Nitori has shown a desire for 'switching' first names with Takatsuki. Towards the end of the series Nitori begins using the name "Lily" at work but it's unknown if that is her preferred name.
Takatsuki: They have known each other since childhood. At one point Nitori expressed her feelings towards Takatsuki but it was not reciprocated at the time. Near the end of the story, Takatsuki expressed her feelings for Nitori, but Nitori had been in a relationship with Anna and could not reciprocate Takatsuki's feelings.
Nitori has brown eyes and brown hair depicted in varying shades, most often a dark tone.
She owns a long haired wig that's a somewhat lighter tone than her natural color and is in a 'hime cut' style. In high school she buys a bob shaped wig of an unknown color. Judging from the shading it might be red. In fifth grade Nitori obtained a sailor suit uniform that used to belong to Takatsuki's older sister.
As a toddler Nitori is shown to have had a medium length, bob shaped haircut. When shown at an older age, around 5 or 6, she has a bowlcut. For most of the manga and anime Nitori is depicted with short haircut with bangs. In middle school, after the incident where she went to school in a girl's uniform, she grew her hair out into a bob. Later on she cut it into a pixie. It grew out over time and by the end of the manga it's a similar length to her original style, but the bangs are different.
Nitori is often stated to be androgynous looking, to the point where she's able to pass as a girl when she dresses masculine. In seventh grade she begins showing major signs of puberty, such as hitting growth spurts and her voice changing. Nitori looks a lot like her sister, Maho, especially when she had grown out her hair.
Nitori Shuuichi is the younger and only brother of Nitori Maho. The story begins in elementary school where Takatsuki, Chiba, Sasa, and Nitori becomes friends. Nitori becomes increasingly seriously interested in dressing up as a girl. During her visit at Chiba's house, Nitori shows an interest in Chiba's dress that was hanging in her room. Upon being asked if Nitori wanted to wear that dress, Nitori says no, even though she might have subconsciously wanted to wear it.
Eventually, Shuu and Maho both sign up and get accepted as models. Maho asks their mom if Shuu was accepted as "girl" or a "boy", but then she says it doesn't matter anyways, since Shuu looked so girly. Shuu ends up quitting modeling later on, since it had initially been mostly Maho's idea to become a model.
Takatsuki gives Nitori some of her and her older sister's clothes since she says that she claims to hate wearing them. Nitori accepts, much to Chiba's dismay since Nitori did not accept them from Chiba. Sometime during this period they met with the transgender woman, named Yuki, and her partner.
As they graduate, Taktsuki and Nitori are often seen going on "dates". It starts off being together on the train, and then progresses to the point where they walk together in public, appearing as an opposite gender couple (Taktsuki as a boy and Nitori as a girl wearing a wig). If asked, Nitori would say her name is Takatsuki Yoshino, and Takatsuki would say her name is Nitori Shuuichi. During this time, the two became really close and Nitori sees Takatsuki as someone that understands her, since Takatsuki strongly believes that one should wear the type of clothing they want.
One day, Nitori shows up at middle school wearing the girl's outfit, and looks exactly like a girl. Nitori's classmates do not accept her, and start talking behind her back, since to them, Nitori is a pervert and sick because she is a "boy" who likes dressing up as a girl. That day, Nitori goes home early and stops coming to school for some time.
As Nitori gets older, her voice breaks. The signs of puberty become increasingly troubling and prominent as she goes from developing acne, growing taller, and eventually developing a masculine-looking jaw line. At this point, she is no longer able to easily present as a girl, since she had visible masculine features and her voice had broke.
However, Nitori still wishes to be a girl. This is evident in the ending when Nitori tells her girlfriend, Anna, that she wants to be a girl. Much to Nitori's surprise, Anna happily accepts Nitori for who she is, and not which gender she identifies as.