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Mellow Kitties are a species of cats seen in Wario: Master of Disguise. They are found in Carpaccio's Lab during Wario's adventure there. They appear to be cat-like bubbles with a teal design surrounding them. They attack by consuming Wario. Any non-physical attack defeats them, and being tapped by the stylus several times after eating Wario works too. The name "Mellow Kitty" is a pun on Hello Kitty.
While inside a Mellow Kitty, Wario loses experience points for his currently-selected disguise, but he does not lose any HP. He does not, however, drop a level. Therefore, once Wario has leveled all his disguises up to Level 3, this enemy is harmless.
This cat burglar has no interest in the true value of his loot. He just likes to stare at shiny things.