When I write new characters, however minor, I see them from two points of view: what they add to the main story (obvious for minor chars), and what they own story is. Imho any character, to feel real, has to have his own personality and goals. For example, your MC's boyfriend shouldn't be there only as her boyfriend - he surely has his own personality, wants different things, and the way his wants meet those of your MC can create a subplot or interesting conflict or simply make their interaction more real.
I can't quite remember who said that no secondary character thinks of himself as secondary - in his mind he's the MC living his
own story. And if you think about it, that works in real life too.
I understand what you're saying about your MC ending up overshadowed - but if she's your POV character that shouldn't be a problem, it's a matter of screentime after all. She could actually be enriched by the other characters' complexity - because everything is related in a story.
Try giving those characters something that isn't entirely in the MC's sphere - either a quest of their own or a personal issue they must overcome, a troubled past, a reason for being there at all. Maybe you already did this, I dunno, but however simple it may sound - making them interesting is just that, making them interesting people beyond their story-fulfilling roles. And I don't mean superficial quirks that only seemingly define them - I mean complex personalities and stories of their own. They shouldn't overshadow the main stuff if you keep it simple (no need to throw in all the backstory you have - simply
knowing your characters will subtly change the way you write them).
Sorry if this is useless - just my 2 cents. I love minor characters myself, mine are always weird (even weirder than my MC, who is more 'subtle' - in fact it's the weirdness that makes writing them easier for me). But I never let their story unravel completely from past to future - the emphasis is on how my MC changes them, and viceversa, how he influences (and sometimes brings to a closure) their destiny. And they almost never get their own POV scenes (unless they threaten me with death). Hope that helps a little, and good luck.
