So if there’s a franchise with an existing background character and you’ve decided to expend on that character to fully flesh them out, you’re wondering if that could be considered an original character? I think a debate about labels are usually kind of pointless to be honest, but sometimes there may be legal reasons why something like that could be relevant. I’d imagine it goes back to just how well-defined the original background character was in the original work. If it’s “Thug with blue shirt #2” with one line of dialogue like “look out boss, he has a gun!” in the screenplay, then there’s not much of a character there. These things exist on a spectrum.
That said, a word of caution, because the original franchise doesn’t have any obligation to respect your work so Marvel may start a new series based on “Thug with blue shirt #2” give him the ability to talk to insects and now your stuff diverges from their timeline. I have no idea if that would be a problem for you, or if you’d care, but it’s worth being mindful about.