Uncle Rosco is Charlotte Page's uncle. Charlotte first mentioned him in The Space Rock when she said if the big crane can move him out of Charlotte's house. She said he has been in her house since Christmas.
He was also mentioned in Caved In. In that episode, it was also mentioned that he already weighed 400 pounds when he was a kid and the bikes just couldn't take it. Charlotte brings up that he says weird stuff to himself, like "Well, hello hamburger. Yo 'bout to take a trip down to Bellytown."
In Dream Busters when Schwoz showed Ray and Charlotte the bloob that would wake Henry up from his deep sleep Charlotte stated that it looked like something Uncle Rosco would cough up.
In Stuck in Two Holes Charlotte mentioned that he can't fit in a car.