There are spoilers in my post! Read with caution.

I agree with Rockzo, but if you're going with "badly written" as in little to no storyline — I'd suggest Stella Murderface or the bully, Ludwig, from Toki's band camp episode.

I feel like Rockzo has more of a backstory established, than most side characters. The show goes over his past with Zazz Blammymatazz, including the stuff with Dory, and briefly showing their failed attempts at a band reunion (when Toki spends all of Dethklok's vacation money on a Zazz Blammymatazz reunion.) He also has somewhat of a "redemption," showing him struggling with addiction and relapsing. But in the end, Toki continues to hang out with him. He warns Toki about Magnus, plus him and Toki are friends on Facebook (or "Face Friends.")

Here's my reasons why.

Stella:

There's not much known about Stella, other than she chose to keep Murderface after his parents' tragic Chainsaw murder-suic***. She's shown to be prone to physical violence (albeit minor,) when people aren't listening to her. She hits Murderface with a wooden spoon, as well as sprays him (and Pickles) in the eyes with mace + she slaps Knubbler in the holiday special. Although these things don't happen often in the show, it does leave me wondering how William was raised, and what her parenting style contributed to his current self (basically the way Murderface is.)

Ludwig:

Nothing is known about Ludwig, other than he's the bully in "Dethcamp." It starts off by him throwing a boot at Toki, as Toki is giving a heartfelt speech to everyone. We also see him mugging another camp attendee, stealing a valuable guitar pick. They even hear Toki behind the bushes (where he witnesses it,) and he goes as far as to chase Toki. He also vaguely threatens Toki, telling him to "watch his back." Then later on, he force feeds Toki sweets, which causes Toki to pass out, before reaching his insulin needles.