Hayato Kamitani, normally referred to by his surname, Kamitani, is a student at Morinomiya Academy and son of the middle school science teacher, Shizuka. He is the second member of the babysitting club and a member of the baseball club. He is in the same class as Ryuuichi.
Hayato is tall and considered handsome by the girls in his class. He has short black hair and narrow eyes with light green irises, features he inherited from his mother, Shizuka. He is impassive and normally has a deadpan expression, conveying a stern and fierce expression only when he has strong feelings, especially when angry at his younger brother Taka.
He is quiet and rarely shows any other emotion besides annoyance or boredom. He tends not to speak unless its necessary, a trait which he shares with his father, Taizou. It has been noted on numerous occasions that Hayato's bad habit of hitting people on the head comes from Shizuka. He is not prone to angry outbursts but uses baseball batting as an outlet to vent his emotions.
He cares for his family and friends deeply, threatening to send Inui "flying to the sky" if he hurts "Ryuuko-san" (Ryuuichi in disguise as a girl) and also searches for Taka who runs away after his mother refuses to buy him a carp jet. He loves baseball—his biggest wish is to play at Koshien Stadium—and he is good enough to be considered the club's ace (though he himself describes the club as being "weak" repeatedly).
Hayato is the firstborn son of middle school science teacher Kamitani Shizuka and high school chemistry teacher Hebihara Taizou. As a child, Hayato had a somewhat abnormal relationship with his father Taizou as he was a very strict parent who profusely checked his son's teeth for cavities, never spoiled him and demanded Hayato to never address him as "father" in school. Before his younger brother was born, his parents divorced and his father, Taizou, moved out of the household causing Hayato to harbor a hatred for his father for not raising Taka and for not introducing himself as his biological father. However, Hayato displays his father's discipline and advice at times to control Taka's instances of misbehaving.
He considers his younger brother Taka as being loud, brattish, and obnoxious, often annoying him beyond his limitations. Even so, he loves his brother dearly but is distant in showing it. Hayato's affectionate feelings for Taka are shown on rare occasions when he is patient with him. Hebihara notes that Hayato rarely cried as a child in contrast to Taka, who cries often when upset.
He often resorts to violence such as hitting his younger brother on the head in order to resolve the instances when he misbehaves or is simply annoying. Sometimes this is extended to the other children (even Kirin to the disbelief of Ryuuichi) and his acquaintances. This personality trait is seemingly inherited from his mother, despite the fact that she ironically scolds him for resorting towards violence.
He shows a considerate and helpful side when it comes to Ryuuichi and Kotarou, especially when they face an occasional problem and tends to act protectively over the two of them. He is popular with the girls at school who are attracted to his cold personality. In the manga, he receives plenty of love letters and confessions on Valentine's day, though he rejects them all since he sees them as a nuisance.
- Hayato is repeatedly forced to disguise himself as a girl, and referred to as "Hayako" by Usaida to trick Satoru, and later, alongside Ryuuichi, pretended to be miko (shrine maidens). On both occasions, the boys are made to pretend to be girls by Yayoi.
- His habit of hitting people who annoy him is something he picked up from his mother, revealed when she hits him and then a sleeping Usaida.
- He is good at the sciences but really bad at the liberal arts. And he is described as being able to do well (in exams) when he tries hard.
- He doesn't have a sweet tooth, but he'll make an exception for tamagoyaki (sweet egg omelette).
- "Haya" from Hayato refers to "hayabusa", meaning "peregrine falcon".
- Ryuuichi points this out, as Usaida notes how even as birds, Hayato and Taka are siblings (as "Taka" means "hawk").