Yu☆Gi☆Oh! Duel Monsters GX

Synopsis

A new transfer student, Blair Flannigan, joins the Slifer dorm. Jaden notes that the new boy looks a bit like a girl, and seems very interested in Zane. Jaden confronts Blair directly about this, and accidentally pulls his hat off, revealing his long hair and the fact that he is actually a she. Jaden duels with her, wishing to understand her better. Blair begins to take over Jaden's Monsters using her cards. He is able to win and Blair is afraid that he will tell people she is actually a girl. Jaden promises he won't. At this point, Zane shows up and Blair admits she has a crush on him. She also admits she is only in fifth grade. The next day, she leaves the Academy and promises to come back and enroll again when she is older, but now has a new crush on Jaden.

(Source: Wikipedia)