Casero, also known as Ooya, is Milk Chan's landlord. He possesses purple hair, white skin, a distinctly daikon radish-shaped head, and typically wears blue attire. His primary function involves persistently pursuing overdue rent payments from Milk Chan, who consistently lags over six months behind. Milk Chan routinely employs shocking, tricky, or unsettling tactics during these encounters, invariably causing Casero to hastily retreat across the ladder to his adjacent home.

He exhibits an anxious disposition, demonstrated by frantic distress over a misplaced purse, resolved only upon its quick rediscovery amidst considerable commotion. His sexuality is explicitly established as gay within the narrative, specifically disclosed in the Japanese version of Episode 9 ("Financial Decay Peninsula") and referenced openly in later episodes. This aspect occasionally informs Milk Chan's deceptive strategies against him.

Casero's narrative function remains a consistent comedic obstacle centered solely on the ongoing rent conflict. His background, origins, and personal development receive no significant exploration or evolution across the series, its compilations, or announced reboots.