Justine is Valerie's roommate. Although at first glance she seems to be the opposite of Valerie - casual, friendly, and sporty - she shares some hidden similarities with her roommate which can be discovered over the course of their shared daytime events. A few slip-ups by Valerie not long after she transferred to the college have convinced Justine that Valerie is holding some big secret, and she's taken to trying to figure out what it might be.
Justine plays a role in some of Valerie's daytime events, including her games events and some of the Discipline training scenes. Affection can be gained with Justine by participating in events in which she appears. If the player reaches level 15 Discipline and has sufficient Affection with Justine, they can choose to form a bond with her, which unlocks more daytime events with her, and potentially with both her and Valerie.