I recall seeing him in 1 episode. Looking on episode summaries it shows him being in 2 episodes of season 3. & like most non cast members when he's not in the episode he vanishes to who knows where. Guess all the non cast members have their own sewer lair where they hang out, Usagi Yojimbo, Mondo Gecko, & the rest.
It's a case of if you missed a few episodes of the show you'd never have heard of those characters or the others that appeared for 1 or only a few episodes.