Otter

The answer I'm most confident about is Classi, she had a huge role in TFBW and helped resolve the conflict in season 19's arc, and yet I barely ever see any fanart of her!

Another character I almost never see fanart for is Mr. Adams, which wouldn't be that surprising if he wasn't Mike Makowski's stepdad, which I think has so much potential for both silly and serious fan-content. Speaking of vamp kids, I always found that One Grown Ass Man Vampire very memorable and funny too.

Scott Tenorman is featured in the highest rated South Park episode and is absolutely crucial to Cartman's character development, but fanart and fanfic featuring him is... rather sparse. Something something not shippable enough I suppose

Shelley has been getting more attention in the fandom recently, but she's still sometimes forgotten or demonized, especially outside of the tumblr fandom echo chamber. She deserves a spot here.

Alejandro is one of the few one-off characters that I actually think is somewhat underrated when compared to other one-offs, especially the "Foreign Kid" clique. He has surprisingly a lot going on and let's be real here, he's more of a Foreign Kid than Damien.