i dont know much cuz i only watch the dub. it seems like brago is the main enemy to me.
I can only think of Brago being the main enemy, he's made the 2nd most apperances of any Mamono (next to Gash of course)
I suggest downloading the original though, its really great.
Actually.. wait a minute.. I just researched on a Gash Bell fansite,
Brago appears in roughly 35 episodes so far..
Umagon appears in Roughly 75 episodes (and thats missing about 20 episodes, so not sure how many he appears in during those 20 eps)
I havent gotten to the part where Umagon meets his Bookkeeper.. but it sounds like Umagon does sort of become an enemy
EDIT: But both Umagon and Brago are still active after 123+ episodes… so who knows XD
I can only watch up to episode 58 right now.. so I'm not sure, I'll have to post later to update with whats going on.
EDIT 2- I guess the english version is up to episode 27.. because on Cartoon Network... the preview for the new episode shows Danny... and Danny only appears in episode 27..