why Isabella always looks so mad?
even in that wallpaper where she's posing
I could be completely wrong here, but i think it's what some others have already pointed, it's just her temperament. She looks like the "ball-buster" type, we can already tell by her clothes and her abs that she is into working out, a very physical person, probably hates laziness, doesn't like guys who are slobs or the types that peak out in high school/college and dont think about the future until it comes, things like that, would probably like a guy only if he could show her that he can keep up with her and not just physically but mentally, is confident and is aware of the bigger picture.
Again i could be completely off here, but i feel like you can tell a lot about a character just by looking at them, obviously this being a fictional character DrPinkCake can make her whatever he wants her to be, but i think if she were real, the way i described her is probably what would fit her personality the closest.
We will eventually find out.