Writing advice:
If you make a fan fiction about a character, but make that character act out of character and its an AU, are you just stealing their look/past relations whit other characters for your oc?
Whenever you write fan fiction, You have to simply write how would they act if something happened (basically, a what if...?)
For example:
(MHA/BnHA) What if Izuku Midoriya had a quirk? (Very basic, the tag 'Midoriya Izuku has a quirk' has tens of thousands of uses in Ao3)
However, We know Midoriya Izuku is a cinnamon roll, who would never kill someone and will always help the friends he has. Now let's look at another fic, that I like, that makes Izuku Midoriya not his self at all, called "What's in a quirk?"
As someone who has put a lot of time and effort into writing characters as aligned with their canon portrayal as I can muster: You do not have to do any of that. This is fanfiction. Write it as OOC and AU as you want. You're not getting paid, you don't have to answer to anyone, this is a hobby you do for fun. You're creating free content and being kind enough to put it out into the world for others to enjoy. If they don't like it, they can move on. Who cares if it's OOC or not aligned with the canon? The beauty of fanfic is that people can write their own fic if they don't like the way you write yours! If you love how you've written a character, there will be other people who love it too, even if it's OOC. This is fanfiction. Writing OOC does not make you a bad writer, and there is only one "rule" for fanfiction that has ever mattered: Don't like, don't read.