Uncle Matt is Conchita's brother in Rat. Described as a rock of sense, Uncle Matt is a slightly posh middle-aged man of know determined occupation, but one who considers himself an expert on all things. He takes particular pride in pointing out the Latin names of any and all animals. Of all of the family, Matt views the transformation of brother-in-law Hubert into a rat in a more detached manner, going forth on the sociological aspects of rats and spouting pseudo-scientific theories.
Apart from Marietta, he's also most directly opposed to killing Hubert, but mostly for practical reasons: the fact that a drowned rat in a toilet is to be avoided, "whether he's a close relation or not," and the general ramifications of patricide (or even ratricide). Uncle Matt proposes sending Hubert to Mulligan's maggot factory, where he can enjoy himself in a rat paradise, and leave a spare suit and bus ticket should he change back.