Thanos is the main antagonist of the Infinity Saga of the Marvel Cinematic Universe. He appears as the overarching antagonist of Phases One and Two and the main antagonist of Phase Three. He serves as the overarching antagonist of The Avengers, and Avengers: Age of Ultron, one of the overarching antagonists of Guardians of the Galaxy. the main antagonist of Avengers: Infinity War and Avengers: Endgame. He is the arch-nemesis of the Avengers and the Guardians of the Galaxy and the adoptive father of Gamora, Nebula and the Black Order.
He is an extremely powerful extra-terrestrial warlord who commands a massive army known as the Chitauri. He formed an alliance with Loki to invade Earth, and later forged a partnership with Ronan the Accuser to destroy Xandar - both in exchange for the deliverance of two powerful Infinity gems, only for the pair to end up failing with their tasks before betraying Thanos afterwards. These two attempts cost him many resources: the majority of his Chitauri and Saakaran armies, his adopted daughters Gamora and Nebula and the Mind Stone. Angry with his subordinates' repeated failure, Thanos began his own campaign to take the Infinity Stones, forcing Eitri the Dwarf to forge the Infinity Gauntlet for him. He and the Black Order slaughtered many populations, planets and people: including the Xandarins, most of the Asgardians and Loki. He was even forced to sacrifice Gamora to obtain the Soul Stone, and came into conflict with the peacekeeps of the galaxy: the Guardians of the Galaxy, the Masters of the Mystic Arts, the Wakandans and most especially the Avengers. He rose above all of them, including Thor's wrath, despite sustaining severe injuries, and performed the Infinity Snap. As half of life perished, he settled on a distant planet and began farming, suffering even more wounds when he used the Infinity Stones to destroy themselves. This lured the remaining Avengers to ambush the weakened Thanos, and Thor angrily beheaded him.
During their Time Heist however, the Avengers unintentionally allowed an alternate Thanos to return to their timeline, 2023, five years after the Snap and the main timeline Thanos' death. Having captured Nebula to do so, he summoned his entire armada of armies to face the Avengers. However, they successfully reversed the Snap and ressurrected half of life, allowing thousands of their hundreds of allies to return and battle Thanos in a devastating manner for one last time. He decided that instead of destroying half of life, he would destroy the universe and create it in his own image, but Iron-Man sacrificed himself as he used the Infinity Stones to perform one final Snap, disintegrating Thanos and his armies and avenging every single person he hurt or massacred.
He is portrayed by Josh Brolin.
His hunger for power, despite already being among the most (if not the most) powerful beings in the universe, is evident by his searching for the Infinity Stones with the apparent goal of controlling all of existence. He desired to use the Tesseract to look beyond the known worlds to find greater ones that it would unveil. He is also extremely dangerous, bloodthirsty, sadistic, and cruel, finding children that he can raise to be assassins and forcing them to watch as he murders their families. He is also an exceptionally intimidating individual, easily striking fear into those who are his allies even if they hold considerable abilities and powers, with Ronan the Accuser showing visible signs of fear in his presence and Loki doing so at the mere mention of him, as well as being able to command the Chitauri army unchallenged.
Thanos is extremely unforgiving and intolerant of failures, with a loathsome nature; The Other warned Loki that he would know a new kind of pain if he did not give them the Tesseract, something which visibly scared the God of Mischief and made him more determined than ever out of fear of Thanos's power. Even though Thor had not yet known about Thanos, he could still sense Thanos's easy control over Loki as being absurd due to Loki's arrogance. This side of his was shown further when Thanos furiously threatened to bathe the stairways in Ronan's blood if he failed to deliver the Orb, something which scared one of the strongest Kree into submission, and forbade Gamora from rescuing Nebula when the latter fell into a trap whilst on a mission in the Cloud Tombs of Praxius. Strangely, while he did ruthlessly kill Loki for failing him and attempting to kill him, he did not torture Loki first, apparently deeming the God of Lies as not being worth his time due to being far more interested in his winning war for the Infinity Stones.
Thanos has little to no regard for his followers, even his adopted children, showing absolutely no empathy when Ronan killed The Other and referring to Gamora as his "favorite daughter", even with Nebula present. He remained calm even after learning of Maw's death and was not fazed by the death of all the Black Order, although given he said Maw did his mission anyway, it is implied he may feel gratitude for their loyalty and respects their efforts. While Thanos's charisma makes him able to inspire complete loyalty to most of his adopted siblings, he is not flawless in his ability to fake parental affection and to the children he dislikes, he inspires no love, as he is despised by his own "daughters", Gamora and Nebula. Thanos has proven to have absolutely no care for Nebula, as he had no qualms removing parts of her in order to make her stronger, taking her apart piece by piece and replacing her with artificial limbs and organs so she would be able to defeat her sister and always punishing her for failing. He even went as far as to cruelly and ruthlessly torture her and seems to have zero respect for her skills as a fighter, going as far as to say that he did not want to waste time killing her. However, Thanos is not completely unable to love. He quickly took intrest in Gamora after seeing her fight against her captor and decided to spare her and even went as far as to shield her from the massacre of the Zehobereis, teaching her on his views of balance. He grew to genuinely care for and deeply love Gamora as his child. Thanos held her abilities above all others and favored her above all of the other adopted childrens, to the point that he entrusted her to find the Soul Stone and was visibly disappointed by her lie. He notably did not torture her but instead Nebula to retrieve the Soul Stone. He was deeply saddened for having to kill her to get the Soul Stone and mourns and feels severe emotional pain for her death that Mantis was able to feel.
Thanos is also manipulative and hesitant to trust, sending Ronan to retrieve the Orb for him and not telling him that it contained an Infinity Stone. He keeps a low profile, lurking in the shadows and making others do his work for him. However, after both of his alliances backfired, he shows that he is fully capable of taking action himself, as he decided to hunt for the Infinity Stones himself.
Thanos is also highly confident to the point of borderline arrogance. He merely smiled when The Other told him challenging the Avengers is like courting Death. He even was unfazed when Ronan threatened to kill him with The Orb, despite knowing well that the Kree was already very powerful even without the Stone, displaying only anger over his betrayal. Thanos also confidently smiled when he decided to collect the Infinity Stones personally.
It has been revealed that Thanos once had a kinder and more benevolent side to himself, as he cared deeply for his planet and people even though he was shunned for being deformed, as he tried everything he could to convince the governments of his planet to do what he suggests. The fact that they were unable to understand his resolve to save his planet and called him insane permanently made him who he was in the present with his desire to exterminate half of the population of every planet so as to save these civilizations from the same overpopulation tragedy which destroyed his home planet. This is why Thanos truly believed his cause for killing off half of everyone in all planets to be a righteous cause, seeing himself a righteous soul who is doing what is necessary to save all the civilizations of the universe, so they do not face the same tragedy as his home planet.
Thanos is also shown to have an honorable side. While perfectly willing to manipulate others, he seems to disapprove of lies, as he never taught Gamora how to lie and was clearly disappointed that she lied on the Soul Stone. He honors his end of the deal, as even though he was angry for Ronan causing him to lose Gamora and returning empty-handed, he was still willing to fulfill the end of their deal. He even extends this to his enemies, sparing Iron Man when Doctor Strange gave him the Time Stone.
Surprisingly, he has shown signs of empathy, benevolence, and respect towards even his opponents. This is first seen when Starlord attempts to kill Gamora due to the promise he made to her, but thanks to the Reality Stone, the ammo was turned into bubbles; it is then that Thanos says he likes Quill for his willingness to kill Gamora when the situation called for it. This respect is seen again when he commends Tony Stark, viewing him as very similar to himself, going so far as to say he wishes humanity remembered him. When Scarlet Witch destroyed Vision, he actually sympathizes for her lose, telling her he has experienced the same kind of sadness when he killed Gamora.