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Gloomius Maximus looks as though he had parts that came straight from a junkyard. His head has a blue paint job and appears to be rusting around his eyes, under his square nose and on his red lips. He wears a red pirate outfit, a golden belt with the letter G on it, a red bicorn hat with a feather on it, a single black boot on his left foot and has a brass sword-like appendage for a left hand and a brass peg leg replacing his right leg.
Gloomius Maximus hated anything fun, and happy in general. According to what he told Zowie, this is because he never had a birthday party of his own as a little bot nor was he ever invited to one, so he decided that if he can't be happy, no one can. So, he began using his un-funerators to suck the happiness and fun out of any bot, object and planet he came across. His arsenal of un-fun weaponry include his Ultra Gloomerator Glum Beam, capable of rendering a person downer than the downy dumps and fall asleep and unable to wake up.
Only the power of laughter is capable of undoing Gloomius' weapons' affects and sending him into retreating. Like all pirate sterotypes, he took anything that's not his, such as when he stole Zowie's Super Silly Ray in his first appearance. He hated laughter, his ownly weakness, as it forced him to retreat. He's very ticklish as seen when he burst out laughing when the Jollies used their swords to tickle him and when Olie threw a Tickle-Me-Pickle at him.
He was immune to fun weapons such as the Super Silly Ray because of his extremely bad mood, but after giving it a tune up, Olie is able to blast countless bubbles at him, and he finally started to feel its effects and acted jovial and giddy. After getting his happy gear running for the first time in so many years, thanks to Olie using the Super Silly Ray and Zowie's act of kindness of inviting him to her birthday party, he became a completely different bot alltogether, declaring, "I'm the happiest bot in the universe!"