Groot

Played by Vin Diesel

Groot is a tree-like humanoid alien from Planet X, a member of a sentient plant species called Flora colossus, communicating almost entirely through variations of the phrase "I am Groot." Capable of regenerating from near-total destruction and regrowing as a sapling with his memories intact, he is fiercely loyal to Rocket and the rest of the Guardians of the Galaxy. Read more →

First turns up in Guardians of the Galaxy.