He is tall and has red, messy hair. He is only ever seen wearing the school uniform with his tie worn loose. On occasions when he helps Kotarou in secret, he wears a paper bag over his head.
He is a brash and blunt person, who openly speaks his mind and acts impulsively. He leaves a bad impression on Ryuuichi initially by calling the daycare children annoying and calling Midori a "poop factory".
He is a bit of a romantic who blushes easily when it comes to Midori's mother, Yukari, and girls that he like in general, and often plays out scenarios in his head with the object of his affections. Despite his childishness at times, he is sometimes mature, as seen of him trying to give up on his crush for Yukari upon learning that she was still married.
Unlike at first appearance, he shows kindness towards children, repeatedly guarding over Kotarou in disguise.
Inui appears in the daycare one day, rudely introducing himself to Ryuuichi and asks to meet Midori, Yukari's daughter. Under the misguided belief that Yukari is a widow, he expects to one day take the place of Midori's father when he eventually marries Yukari. He claims to have "high expectations for the child of the woman he loves", and is extremely disappointed when Ryuuichi reveals Midori is the baby that just relieved herself in her diaper and disgusted him . When Yukari arrives with the news of her husband's return which is confirmed by a bemused Usaida, Inui decides to give up on her half-heartedly. He still has lingering feelings for her, and Usaida manages to bribe him with a picture of Yukari in a bikini in exchange for secretly watching over Kotarou while the toddler journeys through the school to deliver Ryuuichi's forgotten lunchbox.
Inui becomes infatuated with Ryuuichi in a female disguise at the school festival and returns to ask him about the "gentle girl" that he met then. Kotarou immediately points at his brother, which was not picked up by Ryuuichi. Ryuuichi then suggests that it might have been Inomata Maria, and Inui requests Ryuuichi to set a date for him with her which was refused as it might trouble her. Inui claimed that his mind is "going crazy" and that Ryuuichi would not understand his feelings. Ryuuichi relents and promises a date with the "girl".
Kumatsuka Yayoi who saw the situation, claims that it was her responsibility to undo everything that she has done due to the wig and takes charge. Ryuuichi ended up with a wig and a female uniform, looking like a normal schoolgirl. Kotaro is shocked by Ryuuichi's drastic transformation whereas Kirin and Taka are both smitten by it.
The next day, Hayato and Ryuuichi as "Inomata" turn up together to meet Inui. Hayato claims "Inomata" as his girlfriend in order to break Inui's infatuation as part of Yayoi's ploy. Inui is sorry for the situation yet says that he is happy to have met "her". A guilty Ryuuichi decides to reveal that he was the girl Inui met, but trips over a squatting Kotarou and falls in Inui's arms. Inui turns red and stiffens in shock. In the end, Ryuuichi keeps the secret, compelled by how Inui looked like "Kotarou when he believed in Santa" and he "could not bring himself to say it".
- Inui has horrible luck with women. His first love was not, as he thought, a widowed mother but happily married to a husband who takes long work trips. He gets over Yukari only to fall for a girl who is actually Ryuuichi in a wig and girl's uniform.
- He is oblivious to the fact that his classmate Nekota may have a crush on him.
- He states that his type of girl is a soft, cute, fluffy girl. And he doesn't see the girls in his class as actual girls because they are harsh and don't fit his type.
- The "Inu" of Inui means dog.
- ↑ Gakuen Babysitters chapter 53, page 4