Sayaka Igarashi

Secretary of the Student Council and the right hand of Student council president. She is determined to become Kirari's 'Number 1,' and thinks of Yumeko as a plague. Sayaka is intelligent, serious and deeply cares about school and Student Council. She's endlessly loyal to the Student Council President, Kirari Momobami and will do anything for her. It seems that she has a crush on Kirari, as she expressed concern and sadness when she drove off without telling her where she went and what she was doing. While she seems to be reserved and calm, her subduing of a violent student, Jun Kiwatari and telling him what he now owes in the debt recollection game shows that she's every bit as terrifying as the rest of the Student Council.

(Source: Kakegurui Wikia)