Groot

"I am Groot!"

- Groot's catchphrase

Groot is a member of the Guardians of the Galaxy. He is a Flora Colossus from an unknown world.

The Avengers: Earth's Mightiest Heroes
Groot appears along with the rest of the Guardians in "Michael Korvac" searching for the cosmic fugitive Korvac. This version is very large, nearly the size of a tree, and shows the ability to regenerate himself at will.

Ultimate Spider-Man
Groot is an sentient alien plant who spends most of his time within a pot of soil. He appears to have a limited vocabulary, but his team mates can understand him. He can grow into a giant humanoid tree like being with superhuman strength. He is a leading member of the Guardians of the Galaxy alongside Star-Lord, Drax the Destroyer, Rocket Raccoon and Gamora and Nova.

The Guardians battle the Chitauri an alien race led by Korvac. When Rocket Raccoon's ship is pulled into the Chitauri ship, Rocket fakes surrender so that they can free the captive Guardians of the Galaxy members. Afterwards, Spider-Man discovers that the Chitauri are planning to destroy the Earth. The Guardians plan to run the Chitauri ship into the sun while fighting the Chitauri forces. When their tactic does not work, the Guardians end up making their way towards the Dark Matter Cannon. While Nova and Spider-Man head to the Dark Matter Cannon, the Guardians fight the Chitauri and Korvac. After Korvac is defeated by Groot, Nova destroys the Dark Matter Cannon and the Chitauri ship is destroyed.

Guardians of the Galaxy
Groot descends from a race of walking flora and resembles a sentient tree. Despite his strange appearance and limited communication skills, Groot proves to be a valuable ally and a loyal friend—not only to his fellow bounty hunter Rocket but to the rest of the Guardians of the Galaxy as well.

Trivia

 * Late actor Michael Clarke Duncan recorded his lines for Groot shortly before his death in September 2012. According to Brian Michael Bendis, this was Duncan's last job.