The Flash: 10 Best Characters Who Only Appeared In One Episode
1. Music Meister
One of DC's most whimsical characters was given the live-action treatment (as well as a significant power boost) in the second Flash/Supergirl crossover, Duet. Darren Criss lent his vocal and performative talents to a role that was clearly designed to be a silly yet supremely powerful antagonist of sorts.
The interdimensional being enthralled Kara and Barry with the intent of teaching them a lesson on love and the importance of relying on others, even if they're incredibly powerful heroes. That said, the Music Meister was not one to give a simple pep talk, but instead put the two heroes in a hypnotic state that resembled a Old Hollywood-styled movie musical.
The choice to use Music Meister as a villain gave the shows' creative teams a chance to do something different with the straightforward "heroes team up to stop new powerful threat", give the cast a chance to show off their musical talents as well as give fans a light-hearted respite from the gloomy tone of The Flash's third season.