Koyuki is a 15 year old human boy. He is very tall and has flowing, brown hair and brown eyes. Koyuki wears his school uniform, a light sweater with a red tie and brown pants. A white shirt is seen under the sweater. He wears blue and white shoes to match his sweater's collar.
When Koyuki was a child, he was friends with Mahiru and Ryusei. When Mahiru's parents died, Koyuki offered to listen if Mahiru ever needed to talk to someone. He is shown to be very worried and looks out for Mahiru, not unlike the way he is currently. When Ryusei suggests that the three of them play soccer to cheer Mahiru up and gets declined, Koyuki declares that Mahiru hates soccer.
Koyuki is first seen in chapter one preparing for the cultural festival with Mahiru, Ryusei, and Sakuya.
When the four of them are stopped on the street by Berkia, who claims to be a magician, Koyuki is interested and thinks he is cool. His opinion of Berkia soon changes once he attacks Ryusei. After Ryusei is injured, Koyuki holds him in his arms and cries out for help. Berkia then sees that he is alive and asks if he wants to "Join his friend". Mahiru then jumps on top of Berkia to stop him.
The next day at school, Mahiru rushes in, concerned for Ryusei. Koyuki tells Mahiru that Ryusei was injured in an accident. Mahiru tries to explain that a vampire magician attacked them the day before, but Koyuki asks if he is telling one of Sakuya's fairytales.
- He seems to be very fond of Kuro in his cat form. He often calls him cute and offers him cookies.
- He is spooked by Sakuya's vampire stories.
- He likes karaoke.