The alien who looked like an invader and was a refugee, and who then spent years doing a friend's job for him.
Talos is a Skrull, one of the shapeshifting aliens that Captain Marvel introduces as the villains and then reveals as the refugees. Ben Mendelsohn plays him, first in disguise as a S.H.I.E.L.D. agent called Keller, then as himself once he tells Carol Danvers what actually happened: the Kree destroyed the Skrull homeworld and hunted the survivors close to extinction. Danvers agrees to help him find them a new home, and Nick Fury lets Talos and a faction of his people live quietly on Earth in the meantime, helping him protect it.
That arrangement is the thread through his other appearances. He survives the Blip and, with the Skrulls still hunted, lets a million survivors shelter on Earth. In 2024, with Fury off-world, he takes Fury’s place, his wife Soren takes Maria Hill’s, and the two of them are the ones on the ground in Europe when Mysterio stages his fake attacks, and the ones who eventually see through him.
Minor spoiler
By 2026 the Skrull council has thrown him out and replaced him with the radicalised Gravik, who kills Soren, recruits an unaware G’iah, and plans to trick humanity into a war with itself so the Skrulls can take the planet. Talos brings Hill and Fury in to stop him. They fail to prevent a bombing in Russia, where Hill is killed; they do stop an attack on a United Nations plane and save President Ritson from assassination. Then Gravik, disguised as a friendly soldier, kills Talos.