Age: 13
Birthday: September 12
Zodiac sign: Virgo
Height: 154 cm (5'1")
Weight: 45 kg (99 lbs)
Blood type: B
Famous quote: "I don't think anyone can truly understand another person."

Yoshio Kobayashi, known mostly by his last name is a boy who doesn't have much interest in others and is detached from his surroundings, though he does have a friend, Hashiba. He meets Akechi and gets involved in his investigation.

Kobayashi has short dark hair and a feminine face. He is normally seen in his school uniform. He is often confused for a girl due to his slender appearance, which leads to crossdressing. This particular habit begins in episode 3 due to Watanuki targeting young girls.

In casual clothing, he is normally seen in a baby blue dress with dark blue hemming.

He is naturally inquisitive and does not notice the faces of people who are not significant to him. (They appear to him as gradient-colored silhouettes. The color of these silhouettes varies, depending on the palette of the surroundings.) Kobayashi is bored with his life, due to the lack of excitement. He is framed for the murder of his teacher after awakening in the classroom with the murder weapon in his hands and his teacher's mangled corpse set up in the front of the classroom.