Gray

Gray (formerly called MPCTA (short for Merged Planet of Crashed Together Asteroids) is a planet that appears in Season 2 and 3.

Season 2
Gray has formed from the Asteroids in Mass for Sale. He was infected by some kind of virus in Another Solar System Expedition. Gray and the color planets stayed enemies for a long time because

he wanted to join them, but the color planets refused and he had to join the Rocky Alliance because they were more friendly.

Cyan and Blue would apologize to him for bullying him in Parallel Planets.

He was killed by Demon Planets in The End, and was turned into a ghost planet. Luckily, he was swallowed by a black hole.

Alternate Universe
But he came back in 31 part, and in 32 part joined Anti-Venus community,

Season 3
Gray appeared in the first Episode of Season 3 as normal.