I can't believe that this is something people talk about. I've had a group of people in my head for years, complete with the mindscape the reddit FAQ talks about. I just thought I was a little bit crazy; it's nice to see that there's a name for it.

I can't imagine having to deal with just one though. I started with four, which seemed like a good idea when I was eleven, and I found that distracting enough. Having only one sounds like being locked in a small room with only one companion -- I'd rather be in solitary. I kept creating more regardless, and I finally ended up with sixteen (many of those only half-formed, to be fair), before I figured out how to get them to talk amongst themselves and leave me alone. Most are still there (a few seem to have disappeared), I just stay out of that room.

My advice would be to avoid doing this at all, but if you do, create at least two, and give them a nice room (or set of rooms) to stay in with a defined exit. You'll thank me later.