Here's one more for the encyclopedia.
Felix
Performer: (By the voice, I'd say) Jerry Nelson.
Season(s): 31.
Year(s): 2000.
Most Famous Song: "I Love Trash" sung as a duet with Oscar the Grouch. But when Felix sang it, the song was as follows: "Oh I love to Clean! I love to polish and press and perfect things. I love to shine and wash and inspect things."
Description: Though I can't provide a physical description, the character's demeanor was thus.
This character was made to spoof the classic TV show, The Odd Couple staring Jack Clubman and Tony Randall as Felix Unger and Oscar Madison. Felix came to Sesame Street to visit his old roommate, Oscar. However, Felix is a grouch who likes to clean things. The whole episode's plotline revolved around the fight between Oscar and Felix who went to work at cleaning up Oscar's trash can and its contents. In the end, Oscar realized that the two grouches could be friends arguing about leaving the shared trash can messy or clean according to both of their particular living tastes.
Hope this helps.