Isaiah, called Jesaja in the books, is an old, bedraggled persian cat who lives and maintains the Catacombs. He proclaims himself the Guardian of the Dead.
Isaiah is a secretive cats who is more on the timid side. Parts of his demeanor seem disturbing, but he seems ultimately harmless. He is determined to do his job and even the most grisly of murders don't dissuade him from it. Like Joker, he appears to be religious and believe in the martyr, Claudandus.
Isaiah's appearance speaks of days spent inside of Julius Preterius' laboratories for experiments. He is a hard-used cat, with miscolorated patches of fur and a mangled paw and tail. He is grey and very shaggy, vaguely appearing lion-like. Isaiah has yellow eyes and overall appears very elderly.
- He is voiced by Helge Schneider.