Araragi's childhood friend. She is a student with a strong sense of competitiveness which led to her popular nickname, much to her chagrin. Sodachi was the president of a first-year class where both Hitagi and Koyomi also belonged to. At school, she wanted to be called as "Euler", but ends up getting the nickname "How Much", a pun on her surname (お幾ら, o-ikura) and a reference to her personality.

Spoiler

When she tried to figure out the culprit who stole the answers for the mathematics test, everyone voted Oikura as the culprit. She stopped going to school after the incident. In her first year of middle school, she taught Araragi mathematics during summer break, with her being the reason for Araragi being good at math. But she had her motives for doing so. She wished Araragi would notice the domestic violence occurring in her household and for him to report this to his parents, who are cops. Oikura and Araragi had even met when they were in sixth grade when his parents brought her over to stay with them. She ran away from his household shortly after. She later transferred away from Naoetsu High School, having to reintegrate herself with a new school, which turned out to be very difficult for her.