“Massacre Soldier” Killer is a pirate from the South Blue and a member of the Kid Pirates. He is one of twelve pirates who are referred to as the “Worst Generation”.
Following the confrontation with Kaido, some unknown events happened that lead to Killer eating a SMILE and taking on the alias of Kamazo the Manslayer, a street murderer in Wano Country who is secretly an assassin working for the shogun Kurozumi Orochi.
As shown in chapter 944, after eating a faulty SMILE fruit, Killer lost the ability to express negative emotions and is now only able to smile and laugh.
He was sent to the Prisoner Mine at Udon alongside Kid, who previously escaped and got recaptured. Kid questioned him on what Kaido had done to him and the fate of their crew, but Kamazo/Killer could not answer through his laughter and tears. Both were then sentenced by Queen to undergo water torture.
According to Kid, Killer hated his own laugh so much that he beat up anyone who made fun of it. He eventually stopped laughing at all and started to wear a mask to hide his face.
Kid is devastated at the state of his friend to the point that he didn’t worry about the fact of getting captured.
After all, they are longtime childhood friends and founded the Kid Pirates crew together so it makes sense that Kid would share a deep bond with Killer.