Hoshino Sumika

A young idol that Shinji meets by chance while she's in disguise. Although as an idol having a relationship is forbidden it excites her to go out with Shinji. She tries to pretend to be an adult but Shinji quickly realizes that she's underage.

Spoiler

She wants Shinji to ruin her idol career by making her pregnant because the thought of betraying her fans and dancing onstage while pregnant with his baby turns her on.

Her 'favorite number' is 13