Kazuya Nakamine is a supporting character in the anime Horimiya: The Missing Pieces. He is a physics teacher at Katagiri High School and is responsible for managing the school's keys, a duty reflected in his nickname "Key Missing". His role in the series is minor but memorable, as he is one of the faculty members who occasionally interact with the main student cast.

In terms of background, Nakamine is a relatively low-profile staff member. He is always seen wearing a lab coat, fitting for a science teacher, and carries a large number of keys in his pockets. He shares a personal history with two other teachers at the school: he was classmates with the physical education teacher, Sasaki, and he considers the English teacher, Yasuda, a great friend. Despite this friendship, he and Yasuda attended the same high school but do not remember each other from that time.

Nakamine has a laid-back and relaxed personality, but he is not very good at socializing, which leads him to have minimal interaction with students and other faculty members. However, he is not unkind. When he does engage in conversation, such as with the student Yoshikawa, he demonstrates a polite and proper demeanor. His most defining characteristic is his absent-mindedness; the keys he is supposed to safeguard frequently go missing, causing him minor trouble. He has a fondness for cats but is unable to keep one as a pet because he lives in an apartment. His distinctive appearance includes unusual yellow-green hair.

Throughout The Missing Pieces, Nakamine serves as part of the school's backdrop, contributing to scenes that explore the daily lives of the staff alongside the students. He has no notable character development or special abilities, acting instead as a recurring secondary figure whose primary function is to add depth to the school setting through his quirky but harmless nature.