Shisei Houjou

Shisei is a small doll like girl with long silver hair. She wears the standard school uniform, but her sweater is a bit too big for her so it sags over her left shoulder, revealing her shirt.

Shisei is a kind girl who loves her cousin Saito, she will console him whenever Akane gives him a hard time, she has told Saito that if he ever became homeless she would join him on the streets.

Shisei does have a mischievous side as seen when she found out about Saito and Akane's marriage and demanded Hush money from Saito, but this was probably a joke.

She is Saito Houjou's cousin. With a doll-like appearance and a captivating, cool demeanor, she shares a close friendship with Saitou, having grown up with him as if they were siblings. While she acts very simple and affectionate, she also is insightful when Saitou relies on her.

As a part of Houjou bloodline, Shisei was born with high IQ inherited from her ancestors.

(Source: I'm Getting Married to a Girl I Hate in My Class Wiki)