Mettaton is the playable character and main protagonist of Storyswap. After Mettaton falls into the Underground, they embark on a journey to return to the surface. Mettaton is the last of the eight humans to fall into the Underground after travelling to Mt. Ebott. Mettaton is not the "fallen human" named at the beginning of the game. Mettaton's name is only revealed in the True Pacifist Route.
Mettaton is a robot with a human child's soul who wears a striped pink/purple and black shirt, black pants, and pink boots. They have long-length straight black hair, bangs covering their right eye, and a happy smile.
Mettaton's personality is ambiguous, allowing the player to project their personality onto Mettaton. Mettaton speaks to NPCs throughout the game, but the exact dialogue is not shown. NPCs dialogue boxes display "..." to show that the NPC is listening to Mettaton, and some NPCs respond to questions that Mettaton asks.
Depending on their actions, players can interpret Mettaton as merciful or merciless. Some NPCs emphasize Mettaton's kindness, understanding, and grace in the epilogue. Mettaton is obedient and only disallows player input when they follow someone else's instructions, such as when Asriel tells them to hide behind the conveniently-shaped lamp.
Mettaton's determination grants the power to SAVE and heal at SAVE points. The SAVE file can be reloaded to repeat events that occur after it, but NPCs' memories are not completely erased unless a True Reset occurs. This power emerged when Mettaton first awoke in the Ruins.