Mr. Meeseeks

Mr. Meeseeks springs into being from a blue box with one purpose: fulfill the task he's been summoned for, then disappear. He's manic, chipper, and utterly devoted to helping the Smith family with whatever small favor they need, chirping his own name as introduction and encouragement to anyone nearby.

The joke is structural: existence is agony for a Meeseeks until the job is done, so the longer a task drags on, the more his sunny catchphrase curdles into desperation. Roiland's voice keeps the same bouncy cheer throughout, which makes the creeping unraveling underneath so much funnier and, eventually, a little unsettling.