Retsumin Kei, "The Human Calamity", is a powerful assassin of the Black Tiger White Dragon Alliance and got his nickname since he has killed more people than any plague ever has.
Background
Kei has likely killed large numbers of people ever since his early life. He was mentioned by Kaku Shin Ten that when he was ordered to kill his target, who hid in a village, he killed everyone in the village instead of searching for him, just to finish the job.
Personality
Kei is an apathetic master who easily disregards the lives around him, including that of his parrot Pisuke, who he keeps in a cage that he stands on his head. At the time when he was sent to kill Kaku Shin Ten, Chou Yin Lin, and Yo Tekai, he claims it was the "number 17 Pisuke", showing that the previous sixteen all died. They died, because when Kei starts to fight seriously, the gravity force on his body increases, and it'll eventually harm his body, Before that,though, pisuke will die due to the gravity. So each time pisuke dies, Kei knows he has to slow down.
Plot Overview
Kei is one of the many assassins sent to Yokohama Chinatown to kill Kaku Shin Ten, Chou Yin Lin, and Yo Tekkai for defection from the Black Tiger White Dragon Alliance. He quickly gets into a fight with Apachai, and matches him blow for blow, taking a slight advantage because of the parrot on his head. He loses the parrot from his head when Kenichi climbs up his back and knocks it off, and is easily defeated by Apachai afterwards and dragged from the store.
Skills & Abilities
Master Chinese Martial Artist: He is a master-class martial artist who specializes in a style of Chinese Kenpo based on powerful fast thrusts and high speed jabs. He is able to keep up with and overwhelm Apachai during their fight, mostly because of the latter's tendency to hold back when he saw the parrot on his head. While fighting Apachai, he can easily attack Kenichi, who was climbing up his back to remove Pisuke from his head. However once Pisuke is off of his head, he was outmatched by Apachai very quickly and easily.
Master Assassin: His skills as an assassin are incredibly sharp, as he can kill a large amount of people very easily with little effort, and has killed more people than any plague has, which is the reason people call him the "Human Calamity".
Immense Strength: Retsumin possesses great strength as through mere thrusts he's able to puncture through the bodies of normal people and with concentrated attacks, he can match Apachai's strength when the latter was holding because of Pisuke along with leaving fist imprints on the Ryozanpaku master's back.
Immense Speed: He is able to move so fast that the gravity force on his body increases and becomes harmful to himself and others, that's why he keeps a bird in cage on his head as a detector, because when the bird dies from the intensity of his movements, he knows that he needs to slow down.
Enhanced Durability: Kei is able to endure some of Apachai's attacks on his person during their fight with little effect though the Muay Thai master was holding back because of Pisuke and thus when Kenichi took the bird off Kei's head, he was quickly overwhelmed without being able to retaliate.