There was a family on a ship. There was an alien with them, the best friend of the hero.
The family died somehow, and the hero and alien are left alone, stranded on a planet.
At the end of the film the hero finds out the alien killed them on purpose, and was never their friend.
Before this reveal the alien looked gracious, mysterious and kind, but when they fight he takes his cloak off and looks really scary and carnivorous.
I remember that while they're on the planet the hero builds meteor shelters out of shells, but it does seem too similar to Enemy Mine, so maybe my memory stole that part from there.