What's the Best Way to Create Supporting Characters?

I made a thread a little while back and concluded from it that I really just wasn't up for writing. That's more or less still true. I've been a lot happier since that thread, occasionally drawing a character and maybe writing a little biography for them, but always from the perspective that I don't need to go further than that.

However, I still occasionally go back to the story I mentioned in that thread (details aren't important, don't worry), and imagine what it might be like if I did do something with it, and have fun toying with the idea as long as I don't take it too seriously. The reason I'm coming here is because something occurred to me a little bit back that is a large hurdle, and I'm just honestly curious as to how other people feel about it.

As a quick run-down, this story came about when I came up with a character idea I really liked, then I developed the overarching concepts of a setting that would facilitate that character doing the things I wanted to see them doing. To this day, that's still the only things really developed for the story: the main character and where she lives.

I had started working on supporting characters before and still technically consider them canon, but I haven't really done much with them. I knew this was holding back the story, but as I kept writing varying drafts to get a sense for what kind of story I wanted to tell, I realized there was something really important that was missing.

You see, the main character was created as an amalgam of various design ideas and character archetypes I liked. Her world, in turn, was built around that, existing to accommodate her. However, the supporting characters were just added to fill in the gaps, conjured up in order to fill out the story. I realized I had essentially been rushing to put together a story in order to show off this character I had created, and I realized it was hurting my own perception of the character if I dragged them down with a world that I wasn't actually committed to.

The problem I'm running into now is, I don't know how to create supporting characters that I won't feel are just there to fill out the story. I have quite a few character concepts that I'm attached to like the protagonist (though not quite on the same level), but I can't just import them, because each of them are characters I've developed to be the protagonist in their own stories, and they'd end up fighting for my attention.

Just to be clear, this applies to the villain as well. I don't have a set villain yet, but the ideas I have considered feel like they're missing something, and like I'm just trying to create a villain for the sake of it.

I like my main character too much. She's the entire reason any of it exists. It's another thing that still makes me wonder if I should ever try to give her a story. I guess you could say I'm aware she's the Creator's Pet, but I've noticed early enough that it's hard to add anything else to the story to begin with because they just feel like shadows by comparison.

Maybe try playing around with contrasts? Take what you consider the protagonist's defining trait or motivation and twist it or flip it or tackle it from a different angle.