Doctor: "Ah, nice to see you trolls again."
Karkat/Sollux/Vriska/Eridan/Terezi: "W(w)ho the fuck are you."
So for characters I'm thinking the trolls (obviously), then the Reds and Blues, maybe Clay (he can be the only one to remember the last ATTM if I bring him in), and possibly the Crystal Gems from Steven Universe.