Spoilers for Eternals.
Eternals presents a broad group of ancient superhuman beings who came to Earth to protect it from the Deviants. All of them possess the dedication to their mission, but as the movie reveals, some of the characters are braver than others. Not all of them even know what their true mission is, and those that do have different views on it.
That leads to some surprising twists and turns in the new movie, especially for comic book readers who may be fans of the Eternals going back to the original Jack Kirby run in the 1970s. But bravery of many kinds is on full display in the movie, with many characters risking everything for the good of humanity.
Ikaris
The Eternals is one of legendary writer and artist Jack Kirby's most essential works at Marvel Comics. Ikaris is one of his most essential characters, a quintessential superhero. That's why the heel turn Ikaris does in the movie is so surprising. Ikaris ultimately proves himself to be the least brave of the Eternals by murdering his leader and trying to kill the others to allow The Emergence to occur. His blind loyalty to the Celestials and the mission he had been assigned make for some hard choices for him, but he can't be said to be brave.
Sprite
Similarly, Sprite falls down in the bravery department thanks to her loyalty to Ikaris. Sprite actually harbors an unrequited love for the character, which leads her to side with him in the final battle. Sprite even goes so far as to stab Sersei in the back, potentially dashing any hope of stopping The Emergence from happening. Though Sprite contributed to the defense of humanity against the Deviants in the ancient past, this one act of betrayal nearly wiped away all of their good works.
Dane Whitman
Comic book fans know Dane Whitman becomes the Black Knight and one of the bravest members of the Avengers. In the movie, he doesn't get much to do at all and his story is really left for future installments. But Dane does attempt to help out when a Deviant attacks Sersi and Sprite in London. Dane chases after them and helps guide people to safety after the Deviant begins demolishing cars and buses in a busy London street near Camden. He also commits to saving Sersi after the climax of the movie.
Kingo
Kingo initially is very determined to side with Ikaris and the mission of allowing The Emergence to continue. This diminishes his sense of bravery in the movie, especially considering his very self-centered lifestyle as a Bollywood actor/director.
Furthermore, Kingo neglects to participates in the final battle and go up against Ikaris, the Deviants, and become part of the Uni-Mind which ultimately takes down Tiamut, the emerging Celestial that threatens to destroy the world, reducing his bravery significantly.
Ajak
The Celestials are among the most powerful cosmic beings in the Marvel Universe, and outwardly, Ajak is a good soldier. She obeys the orders of Arishem The Judge and proceeds with their mission without question. But Ajak eventually begins to doubt the wisdom of destroying the Earth to give life to a new Celestial. Her doubts don't lead to a lot of action on her part, but she was very brave in questioning Arishem and also allowing the other Eternals to live their own lives. This led to them all gaining their own perspective and agency.
Makkari
Makkari has numerous moments in the movie where she displays her bravery. She is instrumental in the defeat of the Deviants, using her superspeed to pummel the alien creatures before they even know what hit them. She also never gives in against insurmountable odds against Ikaris and The Emergence. She takes much of the lead in the final battle with Ikaris, using her speed to mitigate Ikaris' tremendous powers including flight and laser beams. Even wounded, she never gives up.
Gilgamesh
Gilgamesh proves to be one of the bravest Eternals in the movie. Not only does play a huge role in the defeat of the Deviants over the course of human history, he personally commits to watching out for Thena after she succumbs to the "mad weary," an affliction that turns her into a killing machine. With his strength, he is perhaps the only one of the Eternals to keep Thena in check. He also bravely sacrifices his life when the Deviants attack the Eternals, saving his friends by taking on many Deviants at once.
Thena
Thena is called the goddess of war in the movie and is the greatest warrior among the Eternals. She faces her affliction of "mad weary" with bravery, knowing she could turn on her friends and family at any moment. She fights every single day to hold on to her senses and her memories of the people she loves.
She also takes the lead in the fight against both Ikaris and Kro, the lead Deviant, in the final battle. Despite Kro's tremendous power and her own weakness, she finds the courage to face him and destroy him.
Phastos
Comic fans know Phastos is the engineer of the Eternals and perhaps the least likely to be in the field using the weapons he designed. But he proves to be one of the bravest Eternals in the movie by leaving his family to fight for the world. All Phastos wants to do is live a quiet life in peace, but he puts aside his own wants and fears to help the Eternals develop the Uni-Mind which in the end saves the world from The Emergence. He also contributes to the battle against Ikaris by using his inventions to tie him down.
Sersei
Sersi is perhaps the bravest of all the Eternals in the movie. She overcomes her self-doubt and fear to use her tremendous powers of transmutation to turn Tiamut into solid rock. Before, she could never use her powers against living beings. In the course of the movie, she discovers more about her power and her own self-worth. She puts aside her doubts and leads the team from the brink of disaster to ultimate victory, even though it costs her in the end when Arishem returns to judge the Eternals for their actions.