The Reaper

The God of Death (GoD) is name and title of the world's best assassin.

It is later revealed that he is not the original God of Death and was in fact his disciple.

Appearance
Contradictory to his name, the GoD has a somewhat innocent looking and friendly smile. While disguised, he has short hair and is always seen with his cap. In truth, he has a skinless, skull-like face after intentionally ripping off his own face.

Personality
As the world's best assassin, the GoD is a sadistic person. He has said terrifying things such as dividing Irina Jelavić into pieces, and drowning Korosensei along with all of Class 3-E. On the other hand, he is quite arrogant and egoistical (which proved to be his own downfall).

History
The GoD was born in an extremely wealthy family. One night, his father was killed by an assassin. As the God of Death saw his father's assassination from a window, he admired the assassination. Calling it "a beautiful skill", this motivated him to become an assassin. In Chapter 134, it was revealed that he was originally Korosensei's disciple (who was the GoD at the time) right after Korosensei had finished the job of assassinating his father. Korosensei trained him until the latter betrayed him during an assassination mission. He stole Korosensei's title of the "God of Death" with the result being Korosensei's capture. Chapter 157 further revealed Korosensei spurned his attempts for praise which caused him to realise that he was nothing more than a tool to be polished. At that point he began to cut off his face.

Plot
The God of Death is first mentioned by Lovro when Nagisa asks about the world's best assassin. He made a cameo appearance when some of the students unintentionally injured an old man and called for an ambulance. He next appeared to Class 3-E in the disguise of a florist in order to sell them some flowers, which they would later give to Karasuma to give to Irina Jelavić. He had planted a bug in the flowers to gain Intel on Irina before moving in to kidnap her. After Korosensei left for Brazil, he took the opportunity where there were no teachers available to approach the students. He showed them a picture of Irina in captivity, stating that if they didn't rescue her he would dismember her, and send her back to school piece by piece. This was also where he displayed his inhuman powers, such as natural blending in with the classroom to appear invisible as well as disappearing silently without a trace after throwing some petals in the air.

He later successfully manages to lure class 3-E to his hideout, where he traps them in a cage containing metal bars fused with anti-sensei material, with the goal of also luring Korosensei into the same trap by using the students. The God of Death underestimates the students however, who use a super charged bomb to blow open the side of the wall. He sees this as a warm up, and drops his disguise to hunt down the 27 students. He overpowers and defeats all of them with ease. Along with the help of Irina, who was manipulated by him and thus became his underling, he captures the students and puts them back in his trap, now in handcuffs.

His next goal is to capture Tadaomi Karasuma, but both Karasuma and Korosensei arrive at the same time. However, Irina and the God of Death come up with a back-up plan in which Irina has a collar of explosives around her neck, ready to be detonated by the God of Death. The plan world when Korosensei walks up to help her, she shoots him in his feet, triggering a trapdoor below him in which Korosensei slowly falls. Almost escaping it, the God of Death uses his inhuman skills to shoot Korosensei's tentacles at a speed of mach 2, causing Korosensei to lose his grip and fall into the cage with the students.

The master assassin then reveals his master plan - he would open a floodgate at the end of a tunnel connected to a river, in which the water would stream trough the cage, drowning the students and pressing Korosensei into the anti-sensei cage, cutting him into pieces. If Korosensei tries to make an escape, the God of Death will trigger the explosive collars he put on the students. An enraged Karasuma vows to stop the God of Death, not allowing the students to be killed as collateral damage. The God of Death sees he has no chance in beating Karasuma in his current situation, thus he flees towards the control center to set the water free, along the way, he sets up multipile traps and devices to halt Karasuma's chase, even telling Irina to hinder Karasuma might he get close. Karasuma however, gets closer to the God of Death as he easily escapes his multitude of traps such as an explosive door, guard dogs equipped with machine guns and a falling gurder set up with a crossbow.

As Karasuma almost reaches the God of Death, Irina interferes, holding Karasuma at gunpoint. But at that moment the God of Death betrays Irina and lets the ceiling collaps on both of them. Irina get knocked out, which the class convinces Karasuma to help her instead of continuing the pursuit. In the meantime, the students and Korosensei hatched a plan which involves camouflaging themselves and sticking to the wall the furthest away from the security camera. The God of Death sets off two of the bomb collars, which result in a dull explosion in the middle of the cage (the bomb collars got rewired and broken off, apart from the aforementioned two, which were thus hidden in the middle of the room). The God of Death sees no point in releasing the water now, wanting to go back to square one and recapturing the students again.

On the way back he gets double-crossed by Irina and gets thrown into a pool of water from a great height with Karasuma. As both men land into the pool, the God of Death's mask comes off, revealing his true face, which is nothing more than a skull and hair as he peeled off his face years ago to be able to disguise as anyone. Karasuma and the God of Death then start their fight, dubbed by Korosensei as "the battle of mankind's strongest". As Karasuma gains the upper hand, the God of Death pulls out his "secret weapon", which consists of throwing a rose in the air and shooting a miniscule rifle in his index finger inside someone's chest and hitting the aorta, which causes someone to bleed to death. He shoots Karasuma and thinks he won, but in reality Korosensei put a tentacle trough the cage on the place where the bullet hit, and gushing out tomato juice from it (which he doesn't even like to drink). The God of Death gets stunned by this and shows a weakspot, giving Karasuma the opportunity to uppercut him in the groin. The assassin gets completely overwhelmed and right after gets kicked into a wall by Karasuma, knocking him out.

The defeated God of Death later gets taken into custody. As the God of Death is tied to a stretcher, Shiro finds him and tells him that he knows the real truth about him and he is needed for Shiro's final plan.

The assassin later learns the truth from Shiro that Korosensei was actually his former master. The two decide to work together to kill their common enemy as the assassin is constantly experimented on to give him new abilities. After accompanying Shiro in witnessing one student's battle with Korosensei, the second God of Death tests his new power by levelling a thirty storey building in just a few seconds.

Abilities and Powers
At first glance he doesn't seem to be very wicked, but this is one of his best skills and assets: the power to appear as a harmless individual. Upon his first appearance, he appeared before Lovro undetected and had him comatose with ease. He was responsible for the decrease in assassins after solely defeating them all himself. He is so powerful that rumors persist that he is actually ten people as opposed to a single one, and instances in the manga imply him to be almost supernatural in power.

One skill unique to him is his control of his bloodlust. He can either hiding it completely along with his presence to "blend" into the background, or release so much that it transforms into a "shroud of darkness". This latter trait blocks visual perception of his body with the only perceivable form being that of an approaching shadow.

He has demonstrated being a powerful and intellectual planner, easily trapping the entire Class E and even Korosensei while compensating for most miscalculations. He was technologically savvy enough that in the place of accomplices, he's able to solely perform multi-person tasks; using a remote to operate certain contraptions around his entrapped hostages, and control doors and bars.

Deceptively he is an extremely powerful hand-to-hand combatant, an unusual trait amongst assassins that is useless in the field 99% of the time. However the other 1% of the time, it is fundamental should an assassin ever find himself in such a fight. The GoD was masterful enough that he could defeat almost every hand-to-hand specialist, including Nagisa, in Class E.

Another of his powerful skills is his marksmanship. He is able to attack targets at long-range while in motion, best exemplified by his sniping of a target who was sitting inside a bypassing train. His shot was so precise that he completely hit all of Korosensei's tentacles as he tried to grab hold of the floor despite their intense speed. This demonstrates his impressive eye-sight as with this skill alone he overwhelmed and trapped Korosensei.

Trivia

 * His technique of becoming a shadow-like being to commit assassinations is similar to some accounts of death where people report shadow people encroaching towards them as they die.
 * Instances in the manga resemble him as either as flower, a skull, or simply a shadow.
 * When described by Lovro, the image used in the manga resembled a man holding a book.
 * The English Funimation dub names the God of Death as the "Reaper", a synonym for the Japanese word shinigami.

Quotes

 * (To Class E students) "It's impossible for humans to reap the God of Death. Do not fear, there is only the God of Death reaping man."