The strongest of them and the one who follows orders, which are two descriptions of the same problem.
Ikaris can fly and project cosmic energy beams from his eyes, and has superhuman strength and durability. He and Sersi were together for thousands of years before he left her.
Richard Madden plays him as the reliable one, which the film needs, because Ikaris is the character the story turns on and it cannot afford to signal it.
Minor spoiler
When he discovers that Ajak means to stop the Emergence, he feeds her to the Deviants and then rejoins the others on Earth as though nothing has happened. He has known what the Eternals are for the whole time, and he did what he was made to do.