A stoic tsundere who has good grades, sports prowess, and a competitive personality. However, she's never satisfied and is always aiming higher.
SpoilerKariu has had feelings for Ohga ever since their time in middle school that she even had matching piercings with Ohga during this time. During the summer vacation, she decides to confess her feelings to Ohga, but he confesses his feelings first and they decide to become a couple after being bashful for a while.SpoilerIn Vol.15 Chapter 225.5, they eventually got married (which now makes her Rena Ooga).