Groot
Groot is a flora colossus and one of the Guardians of the Galaxy. He is introduced in the W-Series What-If episode "What If... a Guardian Fell from the Sky?".
Background
Groot is a walking tree, roughly five feet tall and visibly adolescent, and "I am Groot." is the whole of his spoken vocabulary. What changes is inflection and timing, which is enough to carry a greeting, an admission of fear, a plea not to leave, and love. Decoding it takes years of practice, and Rocket is the one aboard who has put them in.
Personality
Groot is a teenager and commits to the part. He will spend three unbroken hours on a handheld game in the cargo hold, and the little he does say is delivered in a register that writes off every adult on the ship as hopeless. What is underneath the sulk is older than the sulk. He puts himself next to whoever needs him and lets the sitting do the work.
Series History
W-Series
What If
The What-If episodes are alternate timelines and do not affect the main continuity.
In "What If... a Guardian Fell from the Sky?", Groot is in the cargo hold with his game when a rift tears open across the Benatar's path and shears the engineering bay off the hull with Rocket inside it. The adolescence comes off him at once. His branches go out after the bay so fast that Rocket can name only one other occasion he has moved like that, and the three words arrive torn open, begging him not to go. The distance opens quicker than his reach can follow.
The game stays down for the two weeks the crew spends looking. When the Benatar sets down at Pawthorne Mansion, Groot stops in front of Rocket, holds five seconds of silence, and then puts the phrase into a shape that means don't do that again. At the ramp his branch settles onto Rocket's shoulder, and they go up side by side. Out past the rift the sulk is back and so is the game, but he has taken a seat near enough for bark to rest against fur, and he asks after the city in the hologram before telling Rocket it looks nice. Earlier, on the pack's porch, Rocket had reached for those same three words to explain what he made of the one word he heard through their wall every night.
Key Relationships
Rocket
Groot reaches for Rocket first and waits for him longest. Nothing between them requires translation, and neither of them explains any of it.