So I've seen some information on Witches' familiars (good stuff, sweet catches), but it seemed kind of limiting, in a way, since I have an OC that acts like a Witch, but has no familiar. Is it possible for a Witch to not have a familiar, or assign the role onto something/someone, whom they will then rebel against? Is rebellion against a familiar figure completely necessary for a Witch's growth?

Rebellion against a familiar can definitely be a thing, and it doesn’t need to be a literal familiar as you’d expect with a witch, it can indeed be a figure or even a friend. And at times you may have an initial lack of a Familiar as part of the development of the Witch?