Ah... good to know I'm not the only one, then. While I am guessing the iconic characters for certain roles have a sort of townie setting to allow more immersive interaction capabilities, it has been shown this is still possible with characters like Greg. With how the system works, however, the characters who you expect to remain NPC status can either wind up with a doppelganger (like Grim or the Night Wraith) due to the game summoning a random for the role and suddenly you have no idea who the "real one" is.
I believe this happened with the Crumplebottoms as well, but this was because they are set in as a more traditional NPC status. With them, I think if you have unique settings regarding aging and dying or they somehow lost their status because of neighborhood stories, a "new one" will spawn to take their place.
Bonehilda, Patchy, the Tragic Clown, the Flower Bunny... they seem to spawn more along the lines of how job related randoms do, so they really are just randoms running around in costumes. So if any of them die or ends up in a townie household, a new random comes along wearing the costume.
Father Winter is a bit of an interesting one since he always first spawns in as Clement Frost, but if he dies or becomes a townie, a new Father Winter will come along, wearing the suit and having a beard, but he is a completely different guy who can sometimes have completely different looks - like brown skin with blonde hair...
All in all, I really wish these characters were coded to be real NPCs. The same one is always generated with set appearance, set traits, set name to given role and the like...