PuhBuhGu is an "evil" version of PBG created by him during the Tomodachi Life review.
PuhBuhGu is a character that PBG originally created as a villain for his town in Tomodachi Life. The character is occassionally referenced, mostly as a name for the characters used on PBGGameplay.
An actual PuhBuhGu was shown very briefly in the Top 10 Funniest Glitches in Video Games, where he mimes that he wants to kill PBG.
According to SpaceHamster's comment in the Zelda Month 2015 Exclusive SHIRTS! video, PuhBuhGu is the evil version of PBG that kills Link PBG, after complaining that PBG is just reusing the same idea as last year!
PuhBuhGu taunts PBG to play Zelda's Adventure. Eventually, PBG makes it go away after thinking positively about the CDI games.