Valentina is...for lack of a better word...mean. She's cold at best and cruel at worst. It's her hot, fiery temper and her serious nature that make it difficult for more sensitive Pokemon to get along with her. Most tend to avoid her just from her harsh words alone, but experiencing one of her destructive tantrums is pretty scary as well, although most don't tend to stick around that long to see them. If you're mentally sturdy enough to befriend her, you'll realize most of the things she says is all hot air, and simply how she talks half the time. It's only when you truly, truly get on her nerves that she actually means it. It's just best not to piss her off, unless you want something broken or burned.
Growing up, Valentina has always been vilified for her hot temper. So much so, that she simply embraces her mean streak. If you can't handle it, then she expects you to stay out of her life.