He was an alien from Mars who crash landed on Earth and eventually became the friend of news reporter Tim O'Hara.
Martin starts off in the film as someone oblivious to the planet Earth who tries to fit in with the surroundings and was untrustworthy of earthlings.
However after meeting Tim, he develops a friendship with him and discovers that not all earthlings are bad especially when he and his girlfriend Lizzie help him return to Mars. By the end of the film he decides to stay on Earth and move in with them.
Martin's true martian form
Martin with Tim
Martin being helped by Tim to adjust to life on Earth