Robin (voiced by Jerry Nelson) is Kermit the Frog's nephew. He is a youngish frog, probably about ten years old in human terms, but at times he seems wise beyond his years. He has a big heart and a sense of wonder about nearly everything; his fresh, non-cynical take on matters often helps the other Muppets realize what's truly important. Robin resembles his uncle in most ways, although his pupils are round and he has no collar.
Robin, probably the smallest semi-regular Muppet besides Pepe the Prawn and Rizzo the Rat, formed an unusal friendship with one of the largest Muppets, Sweetums. Sweetums, whose fearsome appearance belies a kind and gentle personality, and Robin actually have similar outlooks on life, so it is a mismatch only visually.
Robin made several appearances on The Muppet Show, although very rarely as an on-stage performer. He also made a few appearances on Muppet Babies, where he was a tadpole -- an infant while the others were toddlers. Although Baby Robin never spoke out loud (it's hard to speak underwater anyway), he had an inner voice (Russi Taylor) that the audience could hear. It was on Muppet Babies that Robin was established to be the son of Kermit's (unnamed and unseen) sister.
In A Muppet Family Christmas, Robin was exploring the cellar of Mrs. Bear's farmhouse when he found a Fraggle hole. He called his Uncle Kermit down to investigate. The two entered the tunnel and encountered Gobo, Mokey, Wembley, Boober, and Red, who were as astonished to meet Frogs as the frogs were to meet Fraggles. This historic "first contact" situation surprisingly went unnoticed, as neither Kermit nor Robin made any mention of it to the other Muppets. Robin was given a small yellow pebble as a present from Boober; this pebble had been passed amongst the Fraggles for years. Robin later passed the pebble on to Grover.
It was Robin who got the choice role of Tiny Tim in The Muppet Christmas Carol, by virtue of his uncle playing Bob Crachit.