Mion Sonozaki

From Animepedia

The oldest classmate in Keiichi's class. As the elder, she is the class president and everyone looks up to her as a leader. Her social skills are on par with Keiichi, whom she highly respects as a dear friend and rival (Though it is later shown that she has deep affections for him). She often calls herself ojisan meaning old man. She is also the next-in-line for the head of the Sonozaki household, which holds tremendous influence in Hinamizawa. She is often seen with a holstered airsoft gun.