In the trailer, Kro is seen binding Thena (Angelina Jolie) up in his ropelike golden tentacles. He pulls her closer to him and strokes her face while she resists. He has a humanoid shape, but he’s very large, almost twice as tall as Thena. He’s greenish, covered in sinewy muscle, and has four eyes, two on each side. Kro doesn’t have much faith in humanity’s survival. In voiceover, Thena says to him, “We have loved these people” — meaning humans – “since the day we arrived. When you love something, you protect it.”
“You can’t protect any of them,” he responds.
According to Fandom’s Marvel Database, Kro in the comics is something of an antihero. He’s a general of the Deviants whose powers include longevity and near-indestructibility and shapeshifting — he can change the way his face looks. The other Deviants don’t know about his semi-immortality, and he has used his shapeshifting ability to change his appearance and keep from being found out. In his interactions with humans, he’s often believed to be the Devil. He is the secret lover of Thena, and their relationship has put his loyalties closer to the Eternals than the Deviants on many occasions. They are the parents of twins, who as Eternal-Deviant hybrids are chosen ones, though that storyline may not appear in “Eternals.”