Jeff is a giant worm-like alien that Agent J and T fought in Men in Black II. He was responsible for the death of Serleena.
Men In Black II
Jeff appears at the beginning of the film when Agent J and T walk down the subway streets, when they come across a flower-like antenna atop Jeff's head in the subway system. Jay speaks frendily to Jeff, whilst Tee kicks and pulls Jeff's antenna, despite Jay telling him not to. After that, Jeff becomes angered and comes out of the subway system, taking Tee with him. Jeff slams Tee into the ground then returns underground. Jeff proceeds to hit Jay with his tail. Jay jumps on Jeff whilst going back underground, whilst Jeff slithers. Jay proceeds to inject Jeff with a kind of tranquilizer that had no effect on Jeff. Jay is thrown through the back window, into a train. Jay tells all the passengers to move forward in the wagon but they ignore him. As soon as Jeff starts eating the train, the passengers start to move forward. Jay then threatens to shoot Jeff with a gun, and Jeff stops. Afterwards, Jay leaves the subway and sits with Tee on a bench.
Near the end of the film, Jay, Kay and Laura are chased by Serleena in a ship, into the subway system. Jay gets close to Jeff and drops on the ground and Serleena is eaten. Before Laura leaves Earth, Jeff comes out through the top of a building, roaring in pain, as Serleena survived and taking over his body. When Serleena grows into full size, she bursts Jeff to his death, into pieces, which shocks Agent J, K and Laura.