Sarah is a blue-loving mage who cares little for magic in general and prefers to simply burn stuff to the ground.
She has the lowest possible starting Affection towards Dan/Rosa (middle finger).
She first encounters the player in Drape's classroom, where she asks to join the players party. Allowing her to do so causes Drape to injure her as she's disturbing class.
Once in the hallway after class (you might have to attend 2 more classes) she'll show up and ask for a heal. Saying you know a healing spell and then using Boost on her both affects her Affection. Ignoring her causes her to run after you in the street, but she'll be pissed at you.
She currently has 7 spells, of which 1 is hidden.
With 50 or more affection, it's possible to date her, making her the first datable character added in the game.
To do so, you must play as Dan and have no other girlfriend prior. Once she has 50 affection, you can speak to her in the street and agree to go out with her. This is the only way to get access to her hidden Bubble spell.