How big is your headspace? Do you collect dozens of characters who chatter constantly, or does one stand out and demand all your attention over a long period of time?
I don't have a headspace, but I do have a collection of characters I have RPed and/or written prompts for.
I always think of Harry Sullivan as my main one - I've tried putting others in prompt comms but just didn't have enough to write about to keep them there. But with Harry I have more I want to write than I have time or energy to write. I even have notes about what to write if I can just find the time/right prompt/stop signing up to ficathons.
Jo Grant's my secondary one. I've sometimes written prompts for her and they've always been useful for thinking about her. I stopped because they were mostly the same thing in the end, about her relationship with Cliff. Mind you, that was before we got new canon for her future courtesy of Big Finish. She's the main one in terms of RP because she's so friendly it's easy to throw her at new people and have them get on.
Then I have others that I've tried a bit. Ian Chesterton I played in RS and was never that sure about, especially after the reset. Since then I haven't really known what to do with him, but I occasionally get excited about playing him.
Gene Hunt I got all excited about during the first series of Ashes to Ashes, I think it was. I put him in a prompt comm to try him out and found he was harder than I thought and actually, the series was doing a far better job than I was at looking at the inconsistency between what he feels and what he says. The second series didn't excite me into playing him again, so that's probably the end for him.
Teal'c I've only RPed with at
the_blank_slate and that was sort of on a whim. He's definitely secondary there to Harry and Jo because he's that sociable, he doesn't do small talk and he takes a lot more thought, which doesn't go quite so well with being ill.
Harry Pearce I thought about playing for over a year before I decided to actually play him during the last series of Spooks. It's good that there are people playing other Spooks characters because from playing with them I've decided that I'm only interested in playing him with people he knows and in the usual sort of Spooks setting. And I'm not that interested in prompt writing for him, which is somewhat limiting really.
I watched a lot of All Creatures Great and Small while I was ill and decided Tristan Farnon would be an interesting character to play, once I get over the fact that he's a real person. I haven't done much with him, apart from the odd sixwordstories post and I don't really miss him when I don't, so whether I'll ever do anything with him I don't know.
All of which rambling means that the answer is it's mostly Harry and Jo and likely to stay that way as well. I just don't have the time or energy for more than that and after playing them for over two years I've become attached to them.