every character ever made has specific personality traits – things they love, things they hate, things they always do, things they'd never do, etc. it's what makes a character them, it's what makes them unique.
for example, i would never run around the streets naked, yelling vine references at the top of my lungs. (or maybe i would, i don't have morals.) and bruce wayne would never kill anyone for fun; in fact, that's one of the most commonly known things about him.
it's important that when writing fanfiction, we stick to these personality traits. doing this is often referred to as accurate characterisation. people will either get praised for their characterisation, or dragged for it, and it's better to be on the end in which you get praised.
it's important that we keep our character's personality without overdoing it, because repeatedly mentioning one feature about them can get repetitive and boring. you can always carry on doing the same action, but don't go into detail on said action every time; maybe briefly mention it from time-to-time and see if your readers pick up on how that's a trait of the character.
characterisation has never been my strong point – in fact, i objectively suck at it – so i can't really give much advice on this, but all i'm going to say is don't over-exaggerate their personalities, but don't completely change it, either. you're writing about certain characters for a reason, because you like them, and changing them completely erases the reason why you want to write about them.
i would say that i hope this helped but i know it hasn't!
