Unpopular opinion but I feel like adjusting a characters personality or how their viewed in fanfiction is alright to an extent.
Because most books, shows and movies show all the characters from the protagonist's perspective, so the character may be viewed completely differently to someone else, which works really well with Oc and reader inserts.
For example, Iceman and Maverick.
Throughout the movie Iceman is portrayed in a negative light, because Maverick sees him as competition, someone who doesn't think he is good enough. This continues right up until the end where they see each other differently.
How the qualities they hated before are actually really good and they have a shift in perspective, there we see Iceman in a positive light and this continues into Top Gun Maverick.
Thanks for coming to my little Ted talk.
You're doing good authors! Love you, have a wonderful day!!