Thanos

Post-credits: the first appearance of Thanos.

Pursued for it, he ends up in a prison with Gamora (Thanos's adopted daughter, defecting), Drax (whose family Thanos's forces killed), Rocket (a genetically and cybernetically tortured raccoon), and Groot (a tree). Nebula, Gamora's sister, mutilated repeatedly by Thanos as a punishment for losing to Gamora in combat, begins a slow turn from villain to one of the most important characters in the saga. In Vol. 2 she's hunting Gamora for revenge, then reconciles when Gamora explains she never wanted their father's contests either — and leaves again to hunt Thanos.

Thanos acquired it before 2012 and gave it to Loki inside the scepter to fund the Chitauri invasion, expecting both it and the Tesseract back afterwards.

Thanos, with the Black Order, collects the Stones. Loki was never truly on Thanos's side — coerced, not allied. The Avengers shows him visibly unwell and under pressure; Infinity War confirms it when Thanos reminds him he was given an army and a Stone and failed.

Twenty-three days after the Snap, Danvers locates Thanos on a remote planet. Nebula is heavily cybernetic — Thanos replaced parts of her body each time she lost to Gamora in combat. In 2014, past-Nebula involuntarily accesses present-Nebula's memories and sees the Snap, Thanos's death, and the entire plan.