Mercury is the closest planet from Sol in the Solar System. He is 430º C, which is colder that Venus, being almost twice as close to Sol than her, so he gets picked on it often. He's also the most teased planet of the system by Uranus, so he has a somewhat rough life. He might have been Venus' moon when he was born, getting ejected later on. However he doesn't remember anything, as he was a baby when that happened.
Personality
Mercury is often kind and funny with everyone, except when Uranus comes around, then he's almost like a statue, barely talking or doing anything. He's quite sad about being teased by Uranus, and often asks Earth for tips when he's teased, but Uranus will tease him again, and again, and again. At this point, the other planets are helping Mercury by kicking out Uranus from the Inner Solar System when he teases Mercury. Uranus has recently stopped teasing for no reason, so he's more calm now.
He's lonely most of the time, but he made his first close friend, Eris. She invited him to go to the Kuiper Belt with her in Episode 2.
Appearance
Mercury is very similar to Earth's moon in sorts of looks. He's gray, with lots of craters in his surface. Not much more.