Gruber, sometimes called Groober, appears solely in the anime as a supporting character. He partners villainously with Gaster, acting as his silent accomplice in their schemes. A tabby cat, Gruber rarely speaks unless addressed and never initiates plans. He unquestioningly follows Gaster's lead, often without grasping the details, embodying a passive and compliant role.

He debuts in Episode 2 when Gaster imagines ice cream; Gruber snatches it away, sparking a chase that establishes their dynamic. The duo frequently causes trouble, like in Episode 6, where they use a fake sushi shop as a cover to rob a neighboring bank. Their plan collapses after accidentally destroying the stolen money, escalating chaos that requires other characters to intervene.

In Episode 15, Gruber and Gaster seize an opportunity, stealing Boxy Boy while Parappa is distracted by an urgent bathroom need and unable to guard it. This pattern of opportunistic mischief cements their antagonistic roles. Gruber's role remains unchanged throughout the anime as Gaster's accomplice, with no character development, backstory, or expansion into other media. His design and function are confined solely to the anime's episodic conflicts.