This is about the anime characters. For the monster itself, see Tainted Kat.
In the anime, the Tainted Kat Brothers are twin monsters who learn martial arts from Kato. Eventually, one grows weary of Kato's attempts to teach them humility and patience, and vows to achieve ultimate power on his own. His brother follows, and together, they end up in the employ of Naga.
They end up attacking the Searchers, severely harming most of them and separating Genki and Mocchi from the rest of group. After Kato trains Genki and Mocchi, the duo defeats the brothers, though out of anger of losing, the brothers attempted to kill each other, blaming the other for their failure. Fortunately, before they can accomplish this, Kato stops them and brings them to their senses and they tearfully repent, becoming his disciples again and parting from the group on good terms. Later revealed by Pixie at the final battle with Master Moo, the Tainted Kat Brothers, along with their master Kato, became part of a resistance, led by Jim, fighting against Moo's armies. The brothers and their master donate their courage to the Phoenix in order to help it defeat Moo.