Harahara Sensei

I'll do anything to get my sister back, even if it means going up against the greedy Yakuza. Azusa's dreams of becoming an academic have been dashed, and she is now living as a teacher in the countryside. One day, she learns that her younger sister, her only living relative, is in trouble in Tokyo. Azusa, the older sister, must dive into the shady underworld...!?

(Source: Shonen Jump Plus, translated)