First thing you need to know about Luisa: She is an absolute loyal friend and if you hurt those she likes, you will be in a lot of pain. Second thing: She doesn’t let her friends get away with drama. Luisa only tries revenge on those who deserves it (Hence why Henry is still alive after breaking up with Aria so many times).
Her flower is the Tiger Lily, which symbolizes Wealth and Pride. And well, let’s be honest, Luisa is quite prideful of her looks (and has an instagram account in universe with a ton of followers), and is the most wealthy student at the house. She’s wealthy enough that one would wonder why is she staying there instead of renting her own apartment, but that’s a story for another day.