The Reaper

The Reaper (死神, Shinigami, lit. "God of Death"), is the name and title of the world's best assassin.

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

Appearance
Contradictory to his name, the Reaper has a somewhat innocent looking and friendly smile. While disguised as a florist, he has silver hair, amber eyes, and is always seen with his cap. When working as an assassin, he wears black clothing. In truth, the Reaper has a skinless, skull-like visage after intentionally cutting off his own face. He is also apparently able to use his bloodlust to give himself a black specter-like appearance.

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

Personality
As one of the world's best assassins, the Reaper is ruthless and cunning, and is willing to use any means, no matter how amoral, in order to attain his goals.

The Reaper acts with a complete lack of morality and concern for others. He views the children of Class 3-E as nothing more than obstacles, and was willing to drown them as part of his assassination attempt on Korosensei. His ruthless demeanor still remained once he augmented himself with tentacles, impaling Kaede Kayano without a second thought.

Like his former master, he sports a very amicable smile and is easily perceived to be a kind, harmless person. During the time when the Reaper threatened to cut up Irina in front of the class, his passive demeanor made the class unable to feel any sense of fear (something Karma actually dreaded).

Later chapters would reveal that the Reaper was once a person who admired his master immensely and worked tirelessly to be recognized by his idol. His personality changed once he realized he was nothing more than a tool to him, and thus betrayed his master. However, his wish for his master's acknowledgement still lingered even when he was given tentacles by Shiro and right before his death.

History
Before becoming the Reaper, he was born in an extremely wealthy family. One night, his father was killed by the original Reaper. As he witnessed his father's assassination from a window, he fell head over heels over the act, calling it "a beautiful skill"; in turn, this motivated him to become an assassin under the current Reaper's tutelage. Unfortunately for the young apprentice, the current Reaper spurned his repeated attempts for praise which caused him to realize that he was ultimately nothing more than a tool to be polished. At that point, he decided to betray his teacher during an assassination mission, subsequently stealing his teacher's title as the "Reaper" and allowing him to be captured. To ensure that he would not be recognized as a second Reaper, he carried out his assassinations in the same fashion as his former teacher and maintained the latter's connections.

The backstory that the Reaper would later initially tell to Karasuma (about growing up in a violent and corrupt environment) was fabricated. It would eventually be discovered that this background actually belonged to his former teacher.

The Reaper Arc
The Reaper is first mentioned by Lovro when Nagisa inquires 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 encountered Class 3-E again 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. Unbeknownst to his customers, he had planted a bug in the flowers to gain intel on Irina. The Reaper then moved in to manipulate Irina into aiding him in his upcoming assassination attempt.

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 naturally 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 Reaper 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 even 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 binded them with handcuffs and explosive collars, boasting to them that despite the lack of intel, an elite assassin like himself can bring results.

His next goal was to capture Tadaomi Karasuma, but both Karasuma and Korosensei arrived at the same time. However, Irina and the Reaper came up with a back-up plan in which Irina has a collar of explosives around her neck, ready to be detonated by the Reaper. Attempting to help her, Korosensei is caught falling through a trapdoor. The Reaper uses his inhuman skills to shoot Korosensei's tentacles at a speed of Mach 2, preventing him from escaping and subsequently trapping him with his 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 pushing Korosensei into the anti-sensei cage, cutting him into pieces. If Korosensei tries to make an escape, the Reaper will trigger the explosive collars he put on the students. Karasuma, knowing that when the students were in danger from an assassination attempt, his decision was effectively that of the government's, attacks the Reaper and declares his intention to stop him. The Reaper 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 Reaper as he effortlessly disables and bypasses every trap he encounters.

As Karasuma almost reaches the Reaper, Irina interferes, holding Karasuma at gunpoint. But at that moment, the Reaper betrays Irina and lets the ceiling collapse on both of them. In the meantime, the students and Korosensei hatched a plan which involved camouflaging themselves and sticking to the wall in the security camera's blind spot. Looking with the camera, The Reaper thought 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 Reaper, 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 recapture the students again to use as hostages.

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. The Reaper is grappled from behind by Karasuma and they are both tossed into a pool of water. As both men land into the pool, the Reaper's mask comes off, revealing his true face, which is nothing more than a skinless face with hair, as he peeled off his face years ago. Karasuma and the Reaper then start their fight, dubbed by Korosensei as "the battle of mankind's strongest". As Karasuma gains the upper hand, the Reaper uses his "Hidden Scythe"; he throws a rose in the air and shoots with a miniscule rifle in his index finger into Karasuma's chest, which seemingly kills the latter. Unfortunately for the master assassin, the bullet was caught by one of Korosensei's tentacles. Kicked in the groin and punched into a wall, the Reaper was rendered unconscious.

The defeated Reaper later gets taken into custody. As the Reaper is tied to a stretcher, Shiro finds him and tells him that he knows the real truth about Korosensei and the Reaper is needed for Shiro's final plan. The assassin later learns from him 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 Reaper tests his new power by leveling 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 Reaper 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 Reaper then revealed himself to be a ferocious tentacled monster, and immediately attacked his former mentor. It was clear that the Reaper 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 Reaper 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 Reaper'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 Reaper to the anti-tentacle barrier. While Shiro was humiliatingly decimated by the barrier, the Reaper made some slight resistance until Korosensei knifed him in the chest and rammed him into the barrier. In his last words, the Reaper 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 Reaper's body disintegrated.

Pre-augmentation of the Antimatter tentacles
Disguise and Blending in: The Reaper 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 Reaper, and Karma notes that he was unable to feel any fear from the best assassin in the world.

Bloodlust: The Reaper 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 Reaper 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 close combat 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 close combat is Karasuma.

Mobility: The Reaper 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 shots are so precise that he can hit all of Korosensei's tentacles even when 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 to hack Ritsu's mobile version with no reception, utilize remote explosive collars and intricate camera and gate systems.

Gardening: During training under his mentor, he randomly picked up some flowers as decoration, and he was praised by his mentor that he had good taste. The Reaper used this skill later on when he was under the guise of a flower seller.

The Reaper's Invisible Scythe: The 'Invisible Scythe' is the Reaper's unique secret weapon, in which he shoots a minuscule round from a small, hidden gun inside his hand, usually to the surprise of his target. The Reaper 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 cells 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 the top speed of Mach 40, twice the top 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, the Reaper's tentacles possess enough strength to destroy a 30 story building in seconds.

Korosensei
While under the tutelage of the original Reaper, the person who would later become the second Reaper greatly admired his master's talent for assassination. He was satisfiedwith learning new skills from him, but at the same time also longed for his acknowledgement. Unfortunately, after learning he was only being used as a tool, he now viewed his teacher as an obstacle and left him to be captured, subsequently taking his name as the Reaper.

Upon learning from Yanagisawa that the creature known as Korosensei was actually the new form of his former teacher, his desire to kill him intensified, to the point of voluntarily subjecting himself to be augmented with the antimatter tentacles. However, his wish to be acknowledged still lingered, as shown in his final moments.

Kotarō Yanagisawa
Both the Reaper and Yanagisawa shared a mutual desire to kill Korosensei, and as a result, the Reaper was willing to work with him as his accomplice.

Trivia

 * Instances in the manga resemble him as either as flower, a skull, or simply a shadow.
 * The English Funimation dub names the Reaper as the "Reaper", a synonym for the Japanese word shinigami.
 * Interestingly, every character that has been shown to be killed by the Reaper survived (e.g. Lovro, Red Eye). His Graduation Album profile reveals that he left them alive in as a way to ensure they wouldn't figure he was a second Reaper.
 * 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 Reaper. 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."