When Jimmy Valmer and Timmy return to Lake Tardicaca, a camp for handicapped children, Jimmy unknowingly finds himself to be targeted by his former-drug dealer and leader of the Red Camp Game team, Nathan, who has developed a great dislike for Jimmy due to the fact that Jimmy has led the Blue camp team to victory 3 years running. Aiding Nathan is Mimsy, a big, strong, but dimwitted lackey, who inadvertently causes all of Nathan's plans aimed at defeating the Blue team and killing Jimmy, to backfire.
Nathan's first plan is to put a Black Mamba snake into the Blue Team's canoe. However, his orders to Mimsy are "Put the snake in the canoe", making Mimsy assume that Nathan meant for him to put the snake in their team's canoe, rather than Jimmy's. Similar errors involve a map leading to an Indian reservation, whose Indians kill on sight during the Scavenger Hunt, a shark-mating whistle during the surfing competition (that results in Nathan being raped by a shark), and an exploding ukulele.
Mimsy has small messy brown hair. In addition to his mental handicap, he appears to have strabismus in both eyes. He is overweight to the point of his shirt not being able to fit over his stomach. He wears the standard Lake Tardicaca uniform, a red t-shirt emblazoned with the Lake Tardicaca logo near the right sleeve, and brown pants. Outside of camp, he wears a white button-up shirt and brown suit.
Mimsy is extremely unintelligent due to his mental handicap. He often messes up Nathan's schemes unless Nathan explains them in extreme detail. His lack of intelligence makes him easy to manipulate, as showcased by his loyalty to Nathan, who commonly puts him in dangerous or illegal situations. In "Handicar", he seems to be able to occasionally come up with reasonable ideas, only to be ignored by Nathan, who always responds by slapping him and shouting, "Shut Up, Mimsy!".
- Mimsy's personality is made to match that of the Looney Tunes gangster/henchman Mugsy. In fact, his relationship with Nathan is designed to be like that of Rocky and Mugsy.
- Mimsy appears in the theme song along with Nathan, as of "Poor and Stupid".
- Mimsy and Nathan made a cameo together at the end of the episode, "Black Friday", as they join the PS4 supporters.
- In Bring the Crunch Mimsy is dressed as Jason Voorhees from the Friday the 13th franchise.
- Mimsy and Nathan were intended to appear in South Park: The Stick of Truth, but were cut from the final version. Their models and friend icons can still be found in the game files.
- "Crippled Summer" - Seen as Nathan's sidekick in his efforts to injure or kill Jimmy Valmer.
- "Black Friday" - Seen as part of Stan's army to get discounted PlayStation 4s as part of the mall's Black Friday deals.
- "A Song of Ass and Fire" (cameo) - Seen as part of Stan's PS4-supporting army.
- "Titties and Dragons" (cameo) - Seen as part of Stan's PS4-supporting army.
- "Handicar" - Seen as Nathan's sidekick in his efforts to put Handi Car out of business.
- "Moss Piglets" - Seen as Nathan’s sidekick in order to win the science fair.