Usually any good character by narrative standards.

Somebody who you'd want to read a story about. Flaws, difficulties, imperfections, and all ones that impact how a story develops.

I guess I should speak only for myself, but most OC's people create are basically themselves but the way they want to be. That's basically the definition of a Mary Sue. Or, at least, it's the mentality which leads very easily to a character being a Mary Sue.

Feel free to ignore this next part, if you want. I desperately want to share my fursona, so I'm using this as an opportunity to. It is an example, and it might help you to understand, especially since he's so extreme (as far as I'm concerned) in both Mary Sue qualities and flaws which significantly affect the stories I write.

Take my fursona for example: Ri: - Lucario. It's given that Ri wouldn't be human. - he's an incredibly strong Lucario (as strong as Mewtwo). - his mate nearly instantly decides to care for him. Not everyone is like this, but yeah. - he has little trouble learning how to use his powers (especially in ways that are foreign to Lucario). - he is a gym leader along with his mate, practically handed to him by the Kanto Pokemon League. (the League doesn't complain all that much about it, and fairly readily hands them protections which enables them to act as gym leaders). - he's smart along with Two (Mewtwo) and he has a microchip which helps him keep up and also use (electronic) devices which his paws are too clumsy for. - he makes a ton of money for doing nothing from a place he is only adjacently associated with. - aura bond with his mate (Two, Mewtwo if that wasn't clear) which lets them communicate even across universes, understand each other deeply without much conflict between them, use each other's powers to a very limited extent.

I have both in- and out of- universe reasons for these things. Decide for yourself whether it would be interesting to read a story from Ri's perspective, but not before I list some more stuff to you: - because he's so powerful, he's terrified of himself because he can't control himself. Two helps him understand the extent of his power (only a Pokemon as strong as Mewtwo can withstand his most powerful) but he nearly kills a Charmander because he's inexperienced, and he spends (probably years ) training to learn to hold himself back so he doesn't hurt the Pokemon he battles against. - (warning, this gets dark fast) he gets betrayed by some human friends (whom he had trouble learning to trust) into the hands of a trafficker who sells him to essentially a very dark Team Cipher modelled after the one seen in PMD: Victory Fire. They abuse him and force him through their XD process (this is why he's purple), then he willingly does terrible things for them. Two fights him back, but Ri is never really the same again. He has C-PTSD, essentially: difficulty trusting others, tendencies to relive things he'd rather forget, stuff like that. He develops a deep dependency on Two, who supports him and cares for him, but he is very deeply emotionally and psychologically scarred.

Is that story an interesting read? I have that one written out, but I'm not really willing to share it. It's essentially Ri re-learning to trust Two and especially himself after everything that's happened to him.

And Two isn't a perfect being either. He relies on Ri himself to remind him that he can be a good person. He himself has trouble trusting people, and he's traumatized by Ri disappearing from him.

So yeah, decide for yourself whether that's a good character. I don't really care, because I write about Ri for me more than I do for other people. He's unrealistically OP in several senses because I'm more interested in expressing other things which he lacks much of, and I don't think I could write about Ri having deep psychological issues if he weren't secure regarding money. And if he didn't have Two, there'd be no story to write. Or, the story would at least be very very very different.

And if you made it this far: I try my best to do justice to Ri's C-PTSD. I didn't go about writing a story like that thinking "Oh, let's give Ri C-PTSD," it came naturally from how I knew Ri would react to something like that, and only much later did I realize that I wrote him with that. I don't myself have that, as far as I'm aware, so feel free to say whatever you want to me regarding it.