This hapless cheerleader died from an overly ambitious pyramid formation, but still remains optimistic about life.
Tricia is a fetch. In life, she was a cheerleader who died from an athletic accident, most likely a fall. Her spirit is now trapped in the mirror of the upstairs bathroom of the Kappa Lambda sorority house. It's probable that she died somewhere between Haunting 101 and Facepacks & Broomsticks, as she's not in the mirror in the earlier mission.
Tricia can be unlocked in Facepacks & Broomsticks. In order to leave her mirror (and join your team), she needs to learn how to Clone people. This requires Blair Whelchel - someone who already resembles Tricia - to enter the room and look into the mirror. Because Blair usually visits the other bathroom you need to lure her to the bathroom so she will look into the mirror. Tricia can use her power Ethereal Gift so you can place haunters who can lure Blair like Blue Murder with the skill Obsession and Terroreyes with the skill Intrigue.