Seina Chii is a petite second-year high school student standing at 144cm. She is characterized by her large, bright eyes and short blonde hair. While cheerful, social, and caring like a big sister, she also has a short fuse. Having been teased with the pun "Seina Chii is tiny" since she was little, she flies into a rage whenever she hears it. Despite her small size, she has excellent athletic ability and serves as the ace of the basketball team. She is best friends with Iyo Oki. Their height difference is a staggering 41cm.