Mr. Wu is the character in Men in Black 3. He is an alien and Agent K was hitting him in the face. When Jay was telling Agent K to stop hitting Wu. As a reference to the first Men in Black movie, Wu's dead body was appeared on the wall at his place and he got killed by Boris the Animal.
Kay and Jay go to investigate his resteraunt after Kay recieves a phone call from new Men In Black chief Agent Oh, reporting a non-earthly disease from his patrons. Mr. Wu tries to stall them, first by warmly greeting them and offers them a table, then pretending to not know what they are talking about when they demand to see his tanks in his back kitchen. They eventually force their way in, and notice many health violations of unlicensed alien fish, one of which involves a Spikey Bulba, which seemingly disturbs Kay. Wu insists that the fish is of Earthly origins, but Kay ignores him, snatching off his hat and apron, revealing his true sluggish alien appearance and starts demanding to know who the Spikey Bulba is for, going even as far as to start hitting him in the face with it. However, Wu only stubbornly insists that the Bulba is for nobody in particular, and is just kept around in case anybody wants it. Jay eventually intervenes and reminds Wu of their agreement that only humans be fed fish from no other world except Earth. Wu attempts to make amends by serving them their "favorite noodles," to which they reply in disgust, as they have a clear distaste for his noodles.
After Kay realizes that assassins are posing as common patrons and that his old time nemesis, Boris The Animal, has escaped from prison and is planning his revenge, as well as to reveal his taste for Spikey Bulba. After killing all of the assassins, Kay goes into the kitchen and gets into a confrontation with one of Wu's chefs, only for Kay to kill him when he repeatedly bashes his head in with a pan. He then turns around to see Wu's dead body pinned to the wall, as well as seeing a shadow and hearing a growl going up the stairs.
However, with the 1969 timeline altered, with Boris now dead, it is presumed that Wu is still alive and was taken into custody for his actions.