Agent K

K opens the film dispatching a disguised alien at the border and neuralyzing the patrol officers. When his partner D retires, K neuralyzes him, and then recruits cop James Edwards. With no visible emotion, he teaches J to use the neuralyzer, spot aliens and arrive on time. To save Earth, he lets the Bug swallow him and shoots from inside. Afterward, J neuralyzes him so he can retire.

In 2002, K works at a post office in Truro, Massachusetts, and no longer remembers the Light of Zartha: he neuralyzed himself. J rescues him, and together they stop by pawnbroker Jack Jeebs and a society of tiny beings living in a locker who worship him as a god. When his memory returns, K tells J only the essential: the Light has to leave Earth within an hour.

In 2012, K won't talk about Boris or the ArcNet, the shield he installed in 1969, and that pushes J away. Then Boris travels back in time and kills him. The young K, played by Josh Brolin, meets J, sees the future under threat and attaches the ArcNet to the Apollo 11 rocket. In the end he loses an ally, neuralyzes a boy and lets his partner understand why he kept the secret.

The studio wanted Clint Eastwood, who passed. Per Wikipedia, Jones only accepted after Spielberg promised to improve the script. Tom Selleck, Burt Reynolds, Sylvester Stallone, Al Pacino and Robert De Niro were also reportedly considered. In the animated series, K is voiced by Ed O'Ross and later Gregg Berger.