What is the name of the Muppet character who is a green frog and Kermit's nephew?

The green frog who is Kermit’s nephew is Robin the Frog.

Robin is Kermit’s young, sweet-voiced nephew and a recurring supporting character in Muppet productions. Although he is small and often portrayed as childlike, Robin is not simply a younger version of Kermit. His family connection gives him a distinct place in the Muppet world, where he frequently spends time with his Uncle Kermit and his best friend, Sweetums.

Robin first appeared as Sir Robin the Brave in the 1971 television special The Frog Prince. He later became familiar to audiences through The Muppet Show, where one of his best-known moments was singing “Halfway Down,” a song based on a poem by A. A. Milne. He also played Tiny Tim in The Muppet Christmas Carol, one of his largest film roles.

Rizzo is a rat, Beaker is a nervous laboratory assistant, and Scooter is a human-like backstage assistant, so none fits the family clue. In some later stories, Robin is described as the son of Kermit’s unnamed and unseen sister, but the central canonical relationship remains that he is Kermit’s nephew.