What are some traps to avoid when making fanfics with OCs in them?

I am making (trying to make. Hoping I can make) my first fanfic story based on the show Amphibia. I was heavily inspired by the fanfic Medicated (read it. Even if you don't watch Amphibia, please read it. It is a masterpiece of literature and had me crying at all the emotional scenes) to write one involving my OC Salem.

The problem is this will be my first fic with an OC that shares the stage with the canon main character and I was wondering if there are any pitfalls to avoid. I already have a few rules:

1) no OC x Canon shipping. 2) The OC can be a main character but can't upstage the canon main character. They must have their own journey.

Any others I should know about?