Fun Bobby was Monica Geller's boyfriend on and off in Season 1 and Season 2, portrayed by Vincent Ventresca. He first appears in "The One With The Monkey", when he shows up as Monica's date to her New Year's Party.
He is known and expected by everybody to be the life of the party and very funny, but ironically when he appears on screen, he most often is in some kind of distress and weighs everybody down.
In season 1, he shows up in a flood of tears and reveals that his grandfather has just died and, not being able to get a flight out, decided to show up at the party.
In season 2, Monica gets back together with Bobby, who is revealed to be an alcoholic. Monica successfully helps him give up drinking, but once he does, he is not fun in any way. Chandler nicknames him "Ridiculously Dull Bobby" and Monica begins to drink heavily around him so she can be somewhat entertained by him. Bobby tells her that he thinks she has a drinking problem and breaks it off because he is "not capable of being in a co-dependent relationship right now."
In a 2019 interview, Vincent Ventresca thinks Fun Bobby started drinking again following the breakup to the point where, by 2019, he is on the brink of death (source).