No, I don't think she's evil either. An opportunist for sure, she probably knows her position in the house is really unstable. At first, she was a step daughter with none of the rose blood so if ever the dad decided to ignore her, she'd have nothing left. She chooses to hug that thigh fast. The bro is supposed to be the next heir, so she also endears herself to him, I wouldn't be surprised if she'd slowly tested their boundaries over the years to find exactly how much she could get away with. When MC awakens, she doesn't side with MC but at this point she had seen MC as someone without the will to take over the family. The beast attack was already predestined, but next thing she does is the necklace prank, which fails horribly. She complains to the bro and later whines about losing her dress to go to the ball. But she doesn't seem to act out during MC's power gain, she knows better than to mouth at the new real power. Her next move is to immediately try to hug MC's thigh. From her actions, I wouldn't be surprised if Gabrielle had a system or something demanding her to get affection points. But overall, she's far more calculative than she is childish.
As for the ice guy... Eh, he might have found the old her boring, but he didn't have to go full S on her. I don't know if it's better or worse than ignoring her outright, but it looks like his abuse was mostly verbal/emotional, unlike with MC's family where there was gaslighting and physical abuse on top of all that. Don't know if he grew to love her or if it was just a reaction to losing the rose blood power - if she dies, her power should technically go to her bro next. That puts him in a very bad position - dad and bro can claim that he was responsible for killing their 'precious daughter' and use that to pull him down a peg. It could also be something genuine, but it's hard to tell.
I think she's just cunning, which means she can still be very immature.
OK, Gabrielle hmmm I don't think she's like evil or anything, extremely immature and spoiled, yes, but not entirely evil. I blame everything on the father and Ricardo. Also the ice guy probably ended up liking Hilise in the end, seeing his expression when she dies, or at least he cares for her, but ooof too late, If she lived that time I think there might have been a chance between them, luckily she died. That guy has tons of bad vibes, also he's probably a perverted masochist, why else would he want to get slapped.