Felix Leiter is a major character in Casino Royale, Quantum of Solace, a minor character in Spectre, and set up to be a major character in No Time To Die. He was portrayed by Jeffrey Wright.
At the Casino Royale in Montenegro Leiter plays in the high-stakes poker game against James Bond and Le Chiffre. Unable to beat Le Chiffre, Leiter backs Bond with a $5 million buy in, after Bond misreads Le Chiffre's bluff, under the agreement that the CIA be the ones to arrest Le Chiffre after the game.
Quantum of Solace
Later, Felix and Gregg Beam work with Medrano's regime to ensure American gets oil rights in Bolivia when Medrano becomes president by force. Leiter informs Bond of these dealings at a bar in La Paz, briefly informs him that their time window to talk is 30 seconds and warns him that the Bolivian government is after him. Bond escapes at the last minute just as authorities burst into the bar they're in and eventually goes on to defeat Medrano. M later informs Bond that Leiter has been promoted within the CIA.
Bond mentions Felix Leiter briefly to Lucia Sciarra as a form of protection.
James Bond and James Bond Incarnations