SS is the herald of Galactus, who is a planet destroyer. In the comic, SS travels throughout the universe to find suitable planets for Galactus to come and feed on. Before arriving on Earth, SS wasn't empathetic toward any of the planet inhabitants he encountered, but had a change of heart when he got to know humans. He ended up making a stand for mankind against Galactus, and was destined to be a prisoner on Earth for his actions. Not sure if the movie will follow the same script.

Uhhhh, go EDIG. :)