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 as a florist, he has short light hair (silver in the anime), light eyes (amber in the anime), and is always seen with his cap. When working as an assassin, he wears black clothing. In truth, the GoD has a skinless, skull-like face after intentionally ripping off his own face.

During the final arc, after being injected with a stronger version of the Antimatter tentacles from Shiro. The GoD now has a monstrous appearance, bearing his skull-like face and is twice as tall than Korosensei.

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. In addition, he is quite arrogant and egoistical (which proved to be his own downfall). On the other hand, later chapters would reveal that the God of Death formerly admired his master and secretly wished to be recognized by him. Unfortunately the latter only saw him as a tool back then, leading to the apprentice eventually realizing this and betraying his teacher. His wish for his master to acknowledge him still lingered even when he was given tentacles by Shiro and right before his death.

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.

The backstory that the God of Death first gives to Karasuma is a fake story (born in a poor slum with never-ending horror where the only thing you can trust are money and skills), but this is very similar to Korosensei's story. This likely means that in addition to stealing his master's title, he also tried stealing his story.

The God of Death Arc
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 first takes on the 'Attack' squad, easily taking down combatants like Karma, Okano and Chiba in just a minute after the class dispersed. He also foils Nagisa's nekodamashi attempt by executing an even stronger version, explaining it as he lays completely stunned. After the 'Rescue' squad was done in by a manipulated Irina, he convinces the 'Recon' team to stand down, which they do so under Itona surrendering. He then captured all the students and bind them with handcuffs and explosive collars, explaining to them that despite the lack of intel, an elite assassin like himself can bring results.

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. Karasuma, trusting that the Japanese government will choose to think that lives of the students weigh heavier than Earth's, attacks the God of Death and declares his intention to stop him.The God of Death sees he has no chance in beating Karasuma fast enough for the assassination to work, thus he flees towards the control center to set the water free. Along the way, he sets up multiple traps and devices to halt Karasuma's chase, even telling Irina to hinder Karasuma should he get close. Karasuma however, gets closer to the God of Death as he effortlessly disables and bypasses every trap he encounters such using ukemi to bypass as an explosive door, intimidating guard dogs equipped with machine guns, and using his teeth to bite down on a knife trap.

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. Looking at the camera, The God of Death thinks that the students have escaped and 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, believing that the students have both disarmed the explosives and escaped the cell, sees no point in releasing the water now, wanting to go back to square one and recapturing the students again to use as hostage.

On the way back, he accosts Irina and threatens to kill her if she complains about his betrayal, but unexpectedly gets betrayed himself as Irina was acting as distraction. Karasuma, from behind, grapples the God of Death and tossed him and himself into a pool of water. 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.

Master and Apprentice - Final Boss Time
On the night of the deadline to kill Korosensei (exactly one year since the moon had been mostly destroyed), the hooded God of Death and Shiro penetrated the barrier preventing their target's escape from the Class 3-E mountain classroom. The pair brusquely interrupted Korosensei's "birthday" celebration and prepared to kill him. The faux God of Death then revealed himself to be a ferocious tentacled monster, and immediately attacked his former mentor. It was clear that the God of Death easily overwhelmed Korosensei with his Mach 40 speeds, and his mentor was barely able to defend himself. Shiro soon joined in the fight having injected himself with superhuman abilities.

When Korosensei started to effectively dodge the pair's attacks, the God of Death was ordered by Shiro to attack the helpless students. Korosensei used himself as a shield to protect them, until the same student that battled Korosensei intervened and managed to slightly damage one of the God of Death's tentacles. He immediately intercepted her second strike and impaled her through the chest. An enraged Korosensei goes on the offensive until the master and apprentice were locked together. The former accumulated enough energy to blast Shiro and the God of Death to the anti-tentacle barrier. While Shiro was humiliatingly disintegrated by the barrier, the God of Death made some slight resistance until Korosensei knifed him in the chest and rammed him into the barrier. In his last words, the God of Death told his former teacher that when the tentacles asked him what he wanted, he answered that he wanted to be acknowledged by Korosensei and become like him. It was a sentiment his teacher sombrely returned and apologized for before promising to start over together should they meet "on the other side." With this final exchange, the God of Death was disintegrated by the barrier.

Abilities and Powers
Pre-augmentation of the Antimatter tentacles

Disguise and Blending in: The God of Death possesses an intricate set of disguise skills, being capable of acting as a harmless individual at any time. This is further reinforced by his lack of a face, which he cut out to better facilitate his face-changing disguises.

He is also capable of hiding his bloodlust to the point where he can explain his assassination attempt to Class 3-E and yet not impart any fear into them. Even Nagisa who can discern a person easily, feels at ease with the God of Death, and Karma notes that he was unable to feel any fear from the best assassin in the world.

Bloodlust: The God of Death can project his bloodlust freely 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, which confuses his foes.

Hand-to-Hand Combat: The God of Death himself notes that the first thing he polished was his frontal assault capability, which is an uncommon skill for assassins. He effortlessly takes down the 'Battle' squad of Class 3-E in less than a minute, taking out battle specialists like Karma and Nagisa in seconds. He also displays the highest level of the "nekodamashi" attack to the latter. The only human known to match his prowess in combat is Karasuma.

Mobility: The God of Death is extremely fast, being capable of taking down the whole Battle squad of class 3-E in seconds. The only human known to be able to match this speed is Karasuma, however it can be inferred that Houjou can as well.

Marksmanship: 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 can remove all of Korosensei's tentacles when he is moving at a reduced speed of Mach 2.

Intelligence: 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 he can hack Ritsu's mobile version with no reception, utilize remote explosive collars and intricate camera and gate systems.

Grim Reaper's Invisible Scythe: The 'Invisible Scythe' is the God of Death's unique secret weapon, in which he shoots a miniscule round from a small, hidden gun inside his hand, usually to the surprise of his target. The God of Death uses his understanding of the human body and kinetic vision to place this round inside the target's aorta, causing excessive bleeding from the target's heart. This usually kills the target, but does not take in account of the target's vitality.

Post-augmentation of the Antimatter tentacles

Antimatter Physiology: Due to being enhanced with much stronger version of the Antimatter by Shiro, The GoD's entire appearance is a bigger and much darker version of Korosensei's appearance

Enhanced Speed: After being augmented with tentacles, he was able to move at Mach 40, twice the speed of Korosensei. Unfortunately, the confines of the anti-tentacle barriers hindered his movement and prevented him from achieving their fullest potential.

Enhanced Strength: Unlike Korosensei, GoD's tentacles possesses enough strength to destroy a 30 story building in seconds.

Trivia

 * His technique of becoming a shadow-like being to commit assassinations is similar to some accounts of death where people report shadow people approaching 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.
 * Interestingly, every character that has been shown to be killed by the God of Death survived (e.g. Lovro, Red Eye). It is possible that he never intended to kill them in the first place, rather, simply incapacitate them for the time being and show who is superior. Given his aim this whole time which was to be like his master, the original God of Death, this is very likely.
 * When the tentacles asked the God of Death what he wanted, he answered that he wanted to be acknowledged by his master and therefore be like him.
 * In Episode 8 of the Second season, the God of Death threatens to cut off Karasuma's face and wear it. This may be a reference to the character Hannibal Lecter from the horror movie Silence of the Lambs, where he cuts off a victim's face and wears it to use as a disguise to escape.
 * His voice actor, Nobunaga Shimazaki also voiced Karma Akabane in Assassination Clasroom's VOMIC.
 * In the second live action movie his "Koro-sensei 2.0" form is given to Shiro.

Quotes

 * (To Lovro) "From the day you are born, I am by your side always. Do not place fear, in the name 'God of Death'.
 * (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."
 * (To his tentacles about what he wanted to be, and last words to Korosensei) "I wanted to become someone you would acknowledge... I wanted... to be like you."