Sylvia is Grundgetta's worm friend (or pet) on Sesame Street. She features in episode 3856, where she plans on attending the Wormy Gras parade, dressed as a Grouch in a trash can. To her dismay, she discovers that Slimey has a costume identical to hers, created by Oscar and Baby Bear.
When neither worm wants to attend the holiday festivities dressed like anyone else, their problem turns into an argument, which sparks a situation in which they both begin acting like Grouches. Oscar notes that since this is the only way either of them could successfully be a Grouch -- cause a Grouch can't argue with themselves -- they'll have to go together and continue arguing.
Elated at the notion of being able to attend the parade, the grouchy worms then become happy, spoiling Oscar and Grundgetta's revelation cause there's absolutely nothing worse than 2 happy Grouches.