During the Raira Academy entrance exam, Harima Mika sits between Mikado and Seiji and falls in love with Seiji. She immediately begins collecting information about him. Later Seiji saves Mika and Anri from a gang of bullies. Mika then starts to stalk Seiji.

Spoiler

The day before school starts Mika breaks into Seiji's apartment and discovers Celty's head. Seiji hits her with a baseball bat (smashes her head against the wall in the anime) and calls his sister Namie, she disposes Mika's body and later on arranges plastic surgery on Mika's face to make her look like Celty's head. In vol.7, she reveals to Namie that she has tapped almost everyone even slightly connected to Seiji, i.e. Mikado, Anri, Masaomi and even Izaya. She now knows miniscule details about the main on-goings in Ikebukuro regarding these people, from basics like Mikado's, Anri's and Masaomi's identities, Izaya's handle name in the chatroom, to more obscure businesses like the incident with Vorona and Slon, and even Mikado joining force with Aoba. According to Mika, this is due to her wish to protect Seiji and get him out of troubles, should he feel obliged to do something for the others out of gratitude for what they have done for him. This makes her the one who knows the most in the story, second to Izaya (or possibly surpassing Izaya). Only Izaya seems to know about this. Mika also has plans to find Celty's head.