Karl Custer, aka Uncle Yo, is a standup and improv comedian specializing in geek specific subjects including anime, video games, and comic books.

Uncle Yo's first appearance on Channel Awesome was in The Nostalgia Critic's 200th episode, Ponyo. In the review, he bursts into the Critic's room, presumably about to hurt the Critic for reviewing the movie, but then backs off stating he finds the film overrated compared to Hayao Miyazaki's other works. It turns out he was kidding and proceeds to shoot the Critic and berate him. He also appeared in the Critic's review of Baby Geniuses 2, alongside Brentalfloss, where the two attempt to snap the Critic out of his comatose state brought on by the film.

Uncle Yo also appeared in the Demo Reel episode, "Lost In Translation Bromance Version." Here, he meets Demo Reel director, Donnie DuPre, at an anime convention where Donnie hoped to film a version of the film Lost in Translation.

Custer also played the role of Carl in Dragonbored, a short film as part of Year Five event, The Uncanny Valley.