Muscle is a mutated mockingbird who assumed the role of a minor villain in Teen Titans Go!, along with his pal, Flex. For a long time, he was only in his debut episode "Birds", but has since then made a couple of cameos.
When Muscle first appeared, he was nothing more than a regular mockingbird. He, along with his friend Flex, built a nest inside Titans Tower's chimney. Robin ordered them to leave (they were blocking the chute), but the birds refused, and profoundly mocked him. Robin angrily used birdy-cide to poison the mockingbirds, which caused them to leave.
After they escaped, they both mutated into giant mockingbirds. They instantly gained the friendship of all of the Titans (besides Robin). They moved into the Tower, and for a while they got along. They threatened to hurt Robin numerous times, until he finally moved out of the Tower in disgrace. Once he moved out, the birds became the new leaders, but in turn were very obnoxious, and were starting to annoy the other Titans. The other Titans found Robin, now a lowly individual residing in the chimney, karma surrounding him. They ask Robin for forgiveness though, to which he eagerly gives out. After a speedy assault on Muscle (and Flex), they ordered them to exit.
The birds fought back were temporarily winning against the Titans, until they used the Birdy-cide again. Muscle and Flex mutated into superior beings with giant brains, and told the Titans they no longer wished to fight them. Robin attacked them and kicked them out of the Tower to one-up them anyways, and they flew away, probably forever, but not before leaving an egg.
- Superior Intellect: After evolving, he gained superior intelligence.
- Near-invulnerability: He is nearly invulnerable, and can withstand everything but Birdi-cide.
- Super strength: Has the strength to take on all of the Titans, including the super strong Cyborg and Starfire.
- Laser Vision: He can shoot lasers out of his eyes.
- Sonic Screech: He has been seen to be able to screech loud enough to knock his enemies into submission.
- Birds (debut)
- Some of Their Parts (cameo)
- He is voiced by Scott Menville, the voice of Robin.
- The Wilhelm Scream was heard when he and Flex were mutating.
- His purple mohawk and shades are the same as those worn by the Teenage Mutant Ninja Turtle villain, Bebop.