"Sorry to disappoint you, gorgeous, but my heart belongs to the show!"
Mettacrit is a ghost monster with a yellow-pink gradient color, as well as a fringe covering his left eye. He wears a pink bow, and sports a pair of magical gloves.
Mettacrit has a flamboyant, bombastic, and charismatic personality fitting for his career as a performer. He tends to be overly-dramatic at times, and lives for the thrill of the stage. While he greatly cares for his audience, however, he acts narcissistic and self-absorbed at times. Despite this, he does share the spotlight with Chara on a Neutral and Pacifist Route.
To spare him, you must do the Flirt act multiple times. This will make the crowd love his show, and he says that his audience had never been so big, and thanks Chara.
While he's done performances across the Underground, Mettacrit is most popular among the residents of The City of Old, who don't have much exposure to the Underground's most popular entertainment.
Chara arrives just as Mettacrit has finished one of his shows. Being a big fan of humans, even part of a human fan club, he becomes ecstatic when he sees Chara, and decides to start another show featuring them, starting the miniboss battle.
After the battle, Mettacrit is amazed by the audience's reaction, calling it the best reaction to one of his shows he's had yet. He then invites Chara to come visit him in his home in Crystal Springs before leaving to stop at Grillby's.
In the Genocide Route, Mettacrit is one of the few monsters in the City of Old who have not evacuated or locked themselves inside. He is angry that Chara slaughtered his audience and confronts them in battle. Though as a ghost he doesn't really die (or give EXP), he still acts defeated when his HP falls to 0, and vanishes afterwards.
- Mettacrit is confirmed to identify as male, despite lacking a humanoid body. This is due to him finding the courage to be himself, and achieve stardom. 
- It is also confirmed that TS!Underswap interprets ghosts as "biologically" genderless.
- He does performances in The City of Old, in the city's park area. The content of his shows varies.  
- After Mettacrit's battle at the city's park zone, he appears once again at Grillby's, trying to think what he could do in his next performance after suffering an art block.
- There's a picture of Mettacrit in Grillby's Backyard Barbecue.
- Bringing his HP down to 0, regardless of route, does not give the player any EXP or count as a kill. This is due to him being a ghost monster, as he is incorporeal and cannot be damaged by physical attacks.
- However, in the Genocide Route, he acts as if he has been killed.
- Unlike Napstablook in Undertale, he has a dialogue portrait and soundbite.
- The name "Mettacrit" is the headcanon name given to Mettaton's ghost form in the Storyshift AU. This name is used as the canon name in Team Switched's Underswap.
- I was curious about Mettacrit. They referred to their self as a "he", but aren't the ghosts of Undertale gender neutral, only taking genders based on the body they posses? - Official TS!Underswap Tumblr
- Mettacrit, where can I see you perform regularly? - Official TS!Underswap Tumblr
- Mettacrit, I love your bow tie! Also what are your musicals about? - Official TS!Underswap Tumblr