Marty is a character mentioned by Greg Universe as being his manager while he was a Rockstar. He doesn't make a physical appearance throughout My Little Universe.
History
Season 6
In To Give, Or Not to Give, Greg mentions how Marty was his manager during his time going through Delmarva as a roadie, remarking how "he's dead to [him]".
Season 7
In Diamonds in the Rough, He appears in an old TV commercial ad for Pepe's Burgers, using one of Greg's songs as part of the theme song. Greg is given 10,000,000 dollars due to his copyright over the music used.