Potential candidates to replace Kenny and Butters in Professor Chaos, with appearances BEFORE the episode aired. Starting from the back left:
Dougie, who at this point only had more one appearance in “Two Guys Naked in a Hot Tub”, makes a surprising reappearance, establishing his recurring role as General Disarray.
Mark Cotswolds from “Hooked on Monkey Phonics” in season 3, making a rare cameo. He’s made a handful of background appearances, but they are more rare ‘easter eggs’ than him being a legitimate background character.
Fosse McDonald and Bill Allen, who at this point haven’t had a speaking role since season 1 (arguably season 2). They’ll return for “Lil’ Crime Stoppers” next year and have a prominent non-speaking role in “It’s a Jersey Thing” in season 14.
Loogie/Luigi, the mafia kingpin of the tooth fairy-based underworld in season 4’s “The Tooth Fairy Tats 2000” makes a return here, and even makes it to the next round. Like Mark and Damien, he appears sporadically as an easter egg, though by season 14, Billy Miller is our new kingpin.
Damien Thorn, a fan favorite from season 1’s “Damien” who makes background appearances as an easter egg. For a minute or so he has a rose but doesn’t make it to the second round. He makes a return in South Park: The Stick of Truth.
Dogpoo Petuski, a key background character in seasons 1-3 who’s largely faded in and out since. He speaks for the first time here, as well as later in “It’s a Jersey Thing” in season 14 and South Park: The Stick of Truth.
Pip Pirrup’s already on borrowed time - this is his first speaking role in two years, since before the kids moved up to the fourth grade - and it will be his last until season 14’s “201”.
Jason, who’d previously only been seen in one reused cafeteria shot, makes his first serious appearance here, though he wouldn’t speak until season 7’s “South Park is Gay”
Wendy Testaburger, who’s largely been quiet for most of seasons 5-7 - and indeed, has no lines in this episode either - is eliminated surprisingly earlier.
Bebe Stevens, like Wendy, is largely quiet for most of seasons 5-7 excluding three key episodes, and has no lines in her attempt to befriend the boys here. She’s pretty pissed off when she gets eliminated. She gets her moment with them in “Bebe’s Boobs Destroy Society” just a few eps later.
Quaid and Filmore, in that order, are also trying out as well. Both are largely memorable from season 4’s “Trapper Keeper”, though they’ve had other appearances, including as recent as season 16’s “A Nightmare on FaceTime”.
Ike Broflovski is also present, but doesn’t do anything of substance here. Like Wendy, he’s eliminated surprisingly early on.
Token Black has come a long way since making his debut in his current character in season 4’s “Cartman’s Silly Hate Crime 2000” and is largely the frontrunner for the boys’ fourth friend.
Craig Tucker’s already a fairly sizable character at this point, though he has a surprisingly small role here. Despite being the kids’ main antagonist he’s pretty bad at arguing with them - and doesn’t even flip them off.
Clyde Donovan gets a good little speech in but is another kid eliminated early on. We kind of see him as one of the boys in “Cartman’s Silly Hate Crime 2000” though, replacing Cartman while he’s in juvie.
During season 4, Timmy had skyrocketed to being one of South Park’s most recognizable characters, and indeed, some fans at the time advocated him replacing Kenny when the weekly deaths joke became stale. He makes it to the final round here but ultimately fails.
Jimmy Valmer, who’s practically the sixth main character today, is only making his third appearance in the show here, after his introduction in “Cripple Fight” and a speaking cameo in “How to Eat With Your Butt”. He appears to be a frontrunner until he brings over a gift basket.
Towelie’s also making his third re-appearance and second major one. He makes it farther than you’d expect but fails to stand out much besides wearing a bikini and sucking at guitar.
Tweek Tweak, who’s largely been on the fringe of the recurring kids at this point, making only a couple speaking appearances in season 4 and 5, manages to win out and gets six episodes as the main character. It may come as a surprise that, as a fan favorite since his intro, some fans had advocated him replacing Butters before this episode even aired.