Sera Eguchi is a third-year student at Senriyama Girls' High School in North Osaka and a supporting character in the spin-off manga Saki: Achiga-hen episode of side-A. Sera is a real tomboy. Not only that her body and haircut are like a boy's, she also wears male uniform. Her behavior is very boyish as well. She is so boyish she refers to herself as "ore".

Spoiler

Whenever she needs to go to a match, she is forced to wear a girl's uniform in order to represent her school (which is a girls-only school). Every time she has to do that, she gets very embarrassed and even blushes. Sera is Senriyama's ace last year, when she was second year.