In the movie Brick, we know Laura calmed The Pin and Tug down and stopped them from hurting Brendan more. We know she helped Brendan while he was investigating and looked after him when he was hurt. She also wanted to befriend him. But what was her motive? What did she want from Brendan?
It doesn't seem that she needed to have Brendan on her side e.g. for protection. She didn't need anything from him, but maybe she wanted something from him.
Brendan suggests she was power hungry:
I want you to have been on my side all along, not just trying to get me under your thumb, like Brad and The Pin and Tug.
Since Brick is a hard-boiled noir set in a modern high school, it needs all the classic tropes, the #1 being the femme fatale who attempts to lure and destroy the protagonist. Since this trope arose from cultural anxieties over the changing role of women in society, it was enough (from a screenwriter's point of view) for the femme fatale to just be like that - she just enjoys playing with the protagonist. Perhaps that is all the motivation Laura has.