Micah Bell III is the main antagonist of Red Dead Redemption 2 (2018): a gunman who joins the gang after saving Dutch's life, becomes Dutch's new right-hand man, and turns out to be an informant for the Pinkertons. He is killed in 1907 on Mount Hagen. Peter Blomquist plays him.

Micah comes from a family of criminals: his father and grandfather were also named Micah Bell. He joined the gang about five months before the game after saving Dutch's life, and it was his plan that led to the failed Blackwater ferry robbery.
In chapter 3 Micah pushes Dutch towards the fake truce with Colm, and the shootout in Rhodes where Sean dies. After Guarma, his influence over Dutch grows: he advises abandoning Abigail and leaves John for dead.
Yes. In the mission Red Dead Redemption, Pinkerton agent Milton reveals that Micah has been informing for the Pinkertons. In the final confrontation at the camp, Micah kills Susan Grimshaw, and depending on Arthur's honor and choice, may kill Arthur, as explained on the Arthur Morgan profile.
In 1907, John, Sadie and Charles track him to Mount Hagen in American Venom. Dutch shoots Micah, and John finishes him off. Agents Ross and Fordham find the body during the credits.
The rest of the gang, with their actors, is on our Red Dead Redemption 2 characters page, and every mission on the missions page.