Height: 165 cm
Koyomi, the main character of the story, is a third year high school student who is close to failing every class except math. He is an ambidexter, because he usually uses his left hand, but he writes with his right hand.
Shortly before the story ining spring break by giving it his blood, and became a vampire himself. Although Meme Oshino helped him become human again, there are a few lingering side effects; he can see in the dark and far distances, and he heals faster than the average human. He is still scared of morning sunlight even though it no longer has any effect on him.
In Mayoi Jiangshi, in the alternate timeline, Koyomi is killed by Black Hanekawa which pushes Shinobu to swear to destroy the world. It seems that she truly cares about Koyomi and when she saw Koyomi was alive in the alternate timeline, she started weeping blood. At the end, Koyomi simply states that he will die together with Shinobu in the end.Other characters seems to speculate that Koyomi is a lolicon, particularly Mayoi and Hitagi, with Hitagi thinking that Nadeko is a 'threat' to her. Koyomi is the only person who thinks that his family relationship is bad, with Karen and Tsukihi thinking otherwise. Koyomi, Karen and Tsukihi's parents were also revealed to be police officers.