Saw this exact question being asked in the peepshow subreddit, which I'm not subscribed to but came up in my recommended.
So thought I'd ask it here, my favourite minor character from South Park has to be halfy. He was literally in like 2 episodes and then we never saw him again :'(
Who's yours?
Probably Damian. Always found it strange that Satan became a more recurring character in the show but his son Damien only made one major appearance, a few background appearances and is only ever referred to by name again in Professor Chaos. He even makes a cameo in the movie but never once interacts with his dad. It's clear Trey and Matt just don't have interest in him, which is a shame because I think they could come up with more interesting stories with him.
Also Loogie aka Luigi because I'm a sucker for Godfather references