Ares is the main antagonist of Wonder Woman. He is the fearsome, manipulative and notorious Greek God of War, son of Zeus and Wonder Woman/Diana Prince's arch-nemesis as well as her half-brother. He is bent on wiping out humanity at any cost via World War I. As the God of War, Ares is an extremely megalomaniacal, egotistical, bloodthirsty, and manipulative mastermind of terrorism and war from the most diabolical kind ever imagined. Actually, Ares represents the ultimate personification of destruction, war, anarchy, chaos and death, and sees nothing more or less than wiping out humanity at any cost with the usage of World War I.
He is portrayed by David Thewlis.
Ares was born millions of years ago to Zeus, the King of the Olympian gods, as one of the latter's many godly children. In his childhood period, he came to a point of realization that he has mighty powers and that his role is the God of War.
In some point unknown, Zeus had created the human race and made them in the gods likeness, and Ares saw the race with disdain and saw them "ruin the world day by day." and hoping to wipe them out, actually helped humanity with war, fights, blood-thirst and violence. As a response, Zeus created a new race known as the Amazons which were intended to defend humanity from Ares's influence while also spreading love and compassion.(Even though man is capable of waging war without Ares)
However, Ares violently rebelled and killed other gods for disagreeing with his blood-thirst and extremism. To that end, Zeus was left as the only god and faced against his own son at combat and managed to wound him and temporarily subdued Ares for a while but not before Zeus fathered Diana as a contingency weapon that could one day save humanity by defeating Ares, who is the daughter of Hippolyta, and therefore he created the protected island paradise Themyscira for the Amazons to live, being hidden away from the god of war.
Afterwards, Ares reappeared and succeeded to kill Zeus, but not before he vowed to wipe out humanity.
In 1918, Ares had arrived to the city of London, capital of Britain, where he had used his shape-shifting ability to take a human form, and dubbing himself as Sir Patrick Morgan. Under this identity, Ares managed to hide his true-self and disguised himself as an advocate member in the War Council, only to manipulate both sides of the Central Forces and the Allied against each other, in order to advance his goal to destroy humanity.
Diana, who now received the surname "Prince" because of her being a princess in Themyscira, had arrived with Steve Trevor straight to London, where Ares witnessed her arrival. He had maintained his disguise and pretended not to know his identity.
Afterwards, Steve delivers Doctor Poison's notes to his superiors at the Imperial War Cabinet, including Sir Patrick Morgan whose Ares himself, who is "trying to negotiate an armistice with Germany". Steve believes General Erich Ludendorff (the film's secondary antagonist) will complete and use the gas regardless of an armistice, and Diana concludes Ludendorff is Ares himself and slaying him will end the war.
Steve and Diana decide to travel to the front lines to stop Ludendorff, accompanied by Steve's team: spy Sameer, marksman Charlie, and smuggler Chief. Arriving at the Western Front in Belgium, the group's progress is halted by enemy trenches, until Diana pushes alone through the German lines, rallying the allied forces behind her to liberate a village from German control. Diana and the team celebrate the freedom of the villagers, and Diana grows close to Steve.
Learning that Ludendorff will attend a gala at a nearby castle, Steve infiltrates the party and is followed by Diana, who intends to kill Ludendorff. Steve stops her to avoid jeopardizing the mission to destroy the chemical stores, and they quickly learn that Ludendorff has used the gas to bomb the nearby village with severe consequences, causing countless deaths on the innocent civilians.
Diana is devastated that Steve interfered, allowing the loss of life. In rage, Diana pursues Ludendorff to a complex where the gas is being loaded into a bomber to attack London. Diana fights and slays Ludendorff, but is stunned when his death does not stop the war.
After she had killed Ludendorff, Sir Patrick arrives to the place, and confronts Diana. During their conflict, Morgan finally reveals himself to Diana as the true Ares and he explains her that though he has encouraged them to destroy themselves, humans themselves contain the dark impulse to make war. Then, the meeting between them leads into a lengthy, harsh, extremely destructive and violent battle.
As they fight, Ares attempts to convince Diana that humanity does not deserve to be saved, and reveals that she herself is the weapon of Zeus: his last child. As Ares overpowers Diana, Steve hijacks the bomber containing the gas and sacrifices himself to incinerate it at a safe distance.
In rage and grief, Diana attacks the oncoming Germans and effortlessly beats down all of them. Ares triumphantly throws Dr. Poison's getaway vehicle towards Diana, sending Poison tumbling out, her mask falling away to reveal her gruesome face underneath. Ares says that she is the perfect example of humanity and tells Diana to destroy her. Diana lifts a truck and considers crushing Poison with it, until she remembers Steve's last words to her and his declaration of love for her.
Inspired by Steve's selflessness and his final words, Diana spares Poison and lets her run off. Diana says that humanity is indeed everything Ares claimed, but they are also so much more. She dedicates herself to defending mankind and summons her power to stop Ares and spare humanity. He decides that he must kill her and summons lightning before throwing all of it at her. Her bracelets absorb the lightning, and she says goodbye to her brother and flies up before unleashing the full power of her lightning at the God of War, which blasts through him and obliterates him completely and forcing the remaining German troops retreat immediately after.
Back in London, Diana and the team solemnly celebrate the end of the war as they gaze at a photo of Steve along with the other photos of several soldiers in memory on those who died in the war.