Micky Dolenz and Bobo.
"I'm a Believer" is a song made famous by the Monkees, which was sung by Bobo the Bear on episode 105 of Muppets Tonight.
Bobo sings the song in front of a psychedelic background as a sign of his devotion to guest star Cindy Crawford, in what he calls a "60s retro montage fantasy." He is then joined by Micky Dolenz, the Monkees' drummer. Dolenz complains that Bobo is ruining his song, but all is soon forgiven, and they finish the song together.
Kermit had planned a performance of the song on The Masked Singer, but he was eliminated before he had the chance.