Killer, known by the epithet Massacre Soldier, is a pirate from the South Blue and a prominent member of the Kid Pirates. He is recognized as one of the twelve pirates who form the Worst Generation, a group of notorious rookies with bounties exceeding one hundred million Berry who arrived at the Sabaody Archipelago just before the summit war of Marineford. His bounty was initially one hundred sixty-two million Berry and later rose to two hundred million Berry. Killer serves as the combatant of his crew and is the closest partner to the captain, Eustass Kid.

In terms of personality, Killer often comes across as level-headed and pragmatic, a stark contrast to his captain's volatile and aggressive temperament. While he is certainly prone to violence and does not shy away from combat, he maintains a relatively silent demeanor during fights and focuses on efficiently eliminating his enemies. He is more knowledgeable about historical facts than Kid, occasionally providing context and information to his captain. After the two-year timeskip, this trait became more pronounced, as Killer frequently acted as the voice of reason, advising Kid to be polite during meetings with other pirate captains and successfully preventing conflicts from escalating unnecessarily. Despite his calm exterior, he shares his crew's ambition and ruthlessness, having participated in the subjugation of other pirates in the New World. A deeply personal trait is his hatred of his own laughter; he stopped laughing entirely and wears a full-head helmet to hide his face, and he is willing to kill anyone who mocks his laugh.

His primary motivation is his unwavering loyalty to Eustass Kid and the success of their crew. This loyalty is his defining characteristic, driving his most significant actions throughout the story. When the alliance between the Kid, Hawkins, and On Air Pirates was crushed by Kaido, Killer was forced into a dire situation. He accepted a defective artificial Devil Fruit known as a SMILE and became a hired assassin named Kamazo the Manslayer for the shogun Kurozumi Orochi in Wano Country in exchange for a chance to save his captain. The failed SMILE fruit had the tragic side effect of robbing him of the ability to express negative emotions, forcing him to only smile and laugh, which is a cruel irony given his personal history. His motivation throughout the Wano Country arc is to free his captain and reunite his crew to challenge the Emperors.

Killer's role in the story is primarily that of the steadfast right-hand man and a significant supporting combatant. He was first introduced on the Sabaody Archipelago, where he was briefly seen fighting fellow Worst Generation member Urouge. He later participated in the chaotic battle against the Marines and Pacifista, escaping with the other Supernovas. During the timeskip, he joined Kid in forming a pirate alliance aimed at taking down an Emperor, though their target was Shanks rather than Kaido. When Kaido invaded their base, the alliance was shattered. He then endured a period of servitude as Kamazo, during which he was ordered to assassinate the girl Toko and fought against Roronoa Zoro. After reuniting with Kid in the Udon prison, he became a key ally in the Ninja-Pirate-Mink-Samurai Alliance during the raid on Onigashima. In the final battle against Kaido and Big Mom, Killer fought alongside the other Worst Generation captains and played a crucial role, notably damaging Kaido with a specialized sonic attack and defeating the former ally Basil Hawkins in a strategic duel.

Killer's most significant relationship is with Eustass Kid. They share a bond that goes back to childhood, having grown up together and unified their gangs to form the Kid Pirates. Kid refers to Killer as his partner, and his deep concern for Killer's well-being led him to recklessly attempt a rescue, resulting in his own capture. Killer's loyalty is absolute; he sacrificed his freedom and emotional range to secure Kid's safety. He often serves as Kid's strategist and anchor, keeping his captain focused on their long-term goals. He also shares a rivalry with Roronoa Zoro of the Straw Hat Pirates, as they are the only members of the Worst Generation who are not captains and are both swordsmen. After fighting as Kamazo, Killer claimed that had he been using his proper weapons, the outcome would have been different, establishing a competitive dynamic.

In terms of development, Killer transforms from a relatively mysterious and silent Supernova into a tragically sympathetic figure. His fall from a feared pirate to a suppressed, laughing assassin for a tyrannical shogun highlights the brutal reality of the New World. However, his arc culminates in a triumphant return when he dons his cracked helmet once more, regains his agency, and fights alongside his partner against the Emperors, restoring his role as the Massacre Soldier.

Regarding abilities, Killer is a formidable combatant whose fighting style relies on agility, speed, and acrobatic maneuvers to overwhelm opponents. His primary weapons are known as the Punishers, which are a pair of gauntlets equipped with large, curved, sickle-like blades that can spin at high speeds or be used as hand scythes. Using the Punishers, he can perform techniques such as Kamaa Sonic, which spins the blades at the speed of sound to create a soundwave that bypasses external defenses and damages the target internally. This technique proved effective enough to injure Kaido, who is renowned for his nearly impenetrable durability. He is also proficient in both Busoshoku and Kenbunshoku Haki. After eating the defective SMILE fruit, he lost the ability to swim, but gained no other powers from the fruit. As Kamazo, he wielded a pair of standard scythes, but he is considerably more powerful when using his signature Punishers.