The leader of the Eternals and the only one who can talk to the being that sent them, which is why she is the first to change her mind.
Ajak is the wise and spiritual leader of the Eternals, with healing abilities, and she functions as the bridge between the Eternals and the Celestial Arishem. Because of that link she knows the Emergence is coming, and out of a growing love for humanity she decides to try to stop it.
Salma Hayek plays her for a short time and the whole film runs on the decision she makes off screen, before it starts.
Minor spoiler
Ikaris sees her decision as betrayal and leads her to the Deviants, who kill her. Sersi finds the body, inherits the link to Arishem, and the film becomes a question rather than a mission.