Cassus Fett is the secondary antagonist of the Knights of the Old Republic comic book series. He served as the military advisor of Mandalore the Ultimate during the Mandalorian Wars.
Cassus was the one to suggest that they should open the membership of Mandalorians to all species. One of the most notorious attacks launched by Cassus was the genocide of the Cathar species on their homeworld. This caused Revan to rebel against the Jedi Council and helped the Republic to defeat Mandalorians. During the war, he was even disgusted by Demagol and his actions.