Thanos

Thanos was the main character and main antagonist of Avengers: Infinity War and the main antagonist of the Marvel Cinematic Universe as a whole.

Trivia

 * Thanos is the main antagonist of the Marvel Cinematic Universe because he tortured Gamora and Nebula and turned them into weapons, orchestrated the Chitauri invasion (which Bruce Banner would make clear it was Thanos' and not Loki's fault), and worked behind Ronan, also hunting the Infinity Stones, and was behind Loki’s scheme and ordered Ronan the Accuser to kill Drax’s wife and daughter and also killed all our heroes, including some of the Avengers and most of the Guardians of the Galaxy.
 * Interestingly, all of Thanos' daughters have the long "a" at the end of their names, like Gamora, Nebula, Proxima and all of them have three syllables.