A living planet who spent millions of years looking for company and decided, in the end, that he would rather have everything.
Ego is a Celestial and the father of Peter Quill and Mantis. He rescues the Guardians from the Sovereign’s drones, takes them to his planet, which is an extension of himself, and begins teaching Peter to use the Celestial energy he inherited. Then he explains the rest: he seeded thousands of worlds, and turning those seeds into more of him needs the power of two Celestials.
Kurt Russell plays him warm, which is what makes the film work. The scene is a father and a son throwing a ball, and it stays that scene while it becomes the worst thing in the film.
Minor spoiler
The seedlings needed a second Celestial, so Ego fathered children across the galaxy and killed each one that could not use the power. He also gave Meredith Quill the tumour that killed her, because he was starting to love her and did not want to be held. Peter chooses Yondu over him, which is the film’s whole argument in one line.