The strongest of them, who spent the centuries after the group broke up looking after one person.
Gilgamesh is the strongest Eternal, able to project an exoskeleton of cosmic energy around himself. He has a deep connection with Thena, and after the Eternals split up in 1521 he stays as her guardian across the centuries.
Don Lee plays him warm and unbothered, and the film gives him the domestic scenes rather than the fights, which is why the one thing that happens to him lands.
Minor spoiler
He is killed at Druig’s settlement by Kro, a Deviant that has been evolving, who absorbs his powers and his memories with him.