Yukimura is an effeminate student who claims to be a boy, even to Kodaka's disbelief. Yukimura has deep respect and admiration for Kodaka, and even calls him "aniki" (big bro, often used with gang leaders). Yukimura wishes to learn from "aniki" how to be a stronger and more manly boy.
In Volume 5 it is revealed Yukimura is female. Due to her family really wanting a son, she was made to believe that she was a man from very young. She was even in denial that she was female upon Kodaka telling her so until Rika gave her a "lesson" much to Yukimura's shock at the actual truth.