For those paying close attention, I mentioned a game yesterday - it didn't happen as I got very sick very suddenly. Ugh. I'm mostly better now, but that's what happened to game yesterday.
I've had GM PCs in the past - NPCs that were, basically, an active and involved PC except it was run by me, the GM. Players from my old games will probably remember Comet, Justinian, and possibly others I'm just forgetting now. That puts aside the times when I literally ran a PC even as I GMed - my wizard Neves was like that, since my cousin wanted more people playing and we only had himself and me in that game.
We didn't have one in my last game, although we did end up with some long-term NPCs accompanying the group - notably Vim, who lasted longer than you'd have expected.
I'm not a big fan of GMPCs, yet here we are with Raggi.
The PCs picked him up back in session 3, and despite the clear downsides to having him around, they kept bringing him back on their adventures with them. He kills gnoll children, happily butchers the wounded, kills anything he feels like, greedily chases gold, spend his money profligately, often charges wildly into combat without regard to his allies . . . to sum up, the phrase I use is "cheerfully awful."
The players tried to get him killed off. Their PCs didn't - Sense of Duty for one stopped them doing so - but the players kept hoping he'd get killed. His totally wild combat style seemed destined to kill him. But it hasn't; he's survived many fights that would have killed anyone else.
But he's grown on the PCs, especially after his stellar performance against the armored ogre brothers living at the top of the stairs down to level 2.
These days, the players call themselves "Raggi's Roughnecks" and joke that they are only one of his adventuring groups. The running joke is that when he doesn't make his appearance roll, they'll run into Raggi in the dungeon sometime with some other adventurers . . . "Oh, hey! Wow, is it Sunday? Uh, this is kind of awkward . . . ugh, look, I need to finish this thing - I'll call you guys!"
I still treat him like a general NPC. But it's amusing that he's such a highly respected member of the group. And he's become a GMPC in a game that was pretty much anti-GMPC.