Yassen (originally 'Yasha' Gregorovich) was born in the village of Estrov, in Russia. His father was a researcher of chemical properties and a competent biologist; engaged in research at the university of Moscow. There, he invented a deadlier version of anthrax which was highly contagious - the perfect biological weapon for military use; capable of eliminating enemy troops and even entire villages and cities within days. Despite being unwilling to utilize the properties of the virus for military used, he, along with his wife (Yasha's mother) were exiled to the remote village of Estrov and forced to create transferable quantities of the virus for the russian military; under the facade of producing pesticide for farmers.
An accident at the factory caused the outbreak of the virus which infected the whole village within minutes; yet Yasha's parents were able to reach their son in time and inject him with an antidote which they had created, thus saving his life. Yasha's father was, however, fatally shot in the process and died while Yasha watched. His mother forced him to pack basic rations and to leave Estrov and head for Moscow, moments before helicopters from the Russian military arrived to exterminate the village. It is believed that Yasha's mother, along with the rest of Estrov, died during the ensuing carnage.
He managed to take a train to Moscow, where he stays with a crook called Dima, and his so called 'gang'. Dima has stolen 70 rubles of his, but Yasha knows that the only way he can survive is to live with Dima. One of Dima's gang suggests they try burglary, and Yasha suggests to rob Sharkovsky's house. Sharkovsky comes into the house, and kidnaps Yahsa. Yasha is forced to work for Sharkovsky for three years, with two attempts to escape. After three years, Zelin, the helicopter pilot, gets an 'engineer' to 'fix the helicopter'. He actually kills two people and shoots Sharkovsky, but he didn't die. Yasha escaped with them and they go to Venice.
He ended up being recruited by Scorpia, as he had no identity, making him perfect for the job. He was sent to train on an island called Malagosto, in Venice. His tutor was Oliver d'arc, who later became headmaster after Nye. After an unsuccesful mission in New York, he was then trained by John Rider, in fact an MI6 double-agent, and the two earned a rather mutual relationship with each other. On one occasion, John Rider saved Yassen's life during an assassination in the Amazon, with the only price being a thin, straight scar that could have been drawn with a ruler. This scar would stay with Yassen for the rest of his life.
Yassen finds out that John Rider worked for MI6, as he found a special Power Plus radio transmitter in his bag. Yassen somehow comes to the decision to kill Sharkovsky, and he travelled to the 'dacha'. He kills Sharkovsky for good, then kills Ivan too.
He becomes a succesful assasin for Scorpia, and kills many people, including Ian Rider. On one mission he finds Alex and decides to let him live, after what happened to John Rider. Alex promises to kill Yassen.
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Later, after several successful missions, Yassen and John Rider were sent to Malta for an assassination, which turned out to be an MI6 stakeout. In the ensuing gun battle, Yassen shot several agents and wounded Ash. But he failed to kill Ash in his escape.
After John Rider's death, Yassen grew up to be one of the world's most efficient and cold blooded assassins and earned a reputation for being an assassin who never fails. It is suggested that the number of his victims exceed a hundred. In Stormbreaker, he was hired by Herod Sayle to transport smallpox virus to Cornwall. He was dispatched by Sayle to murder Ian Rider. He did so after an exciting car chase, by hanging from a helicopter and shooting Ian Rider with two Grach Russian pistols.
Later, he is seen transporting the viruses by Alex, where he shoots one worksman for denting one of the containers, claiming his people do not like mistakes. He then shoots Herod Sayle after the operation for which Sayle had hired Yassen failed, keeping Scorpia's anonymity intact. He, for the first time, meets Alex Rider, and Alex vows to take his revenge. Yassen subsides this threat and demands that Alex go back to school, for he didnt belong to the same world as him.
Yassen Gregorovich is killed when Damian Cray shoots him in Eagle Strike. Yassen tells Alex that he knew his father, and that he was working with him. His last words are, "Go to Venice. Find Scorpia. And you will find your destiny."
Yassen, to keep his conditions intact and to assist him in his upcoming missions, individually practices marksmanship, works out in the gym or goes for a run. He does this at night and only sleeps for at most four hours.
Extremely skilled, highly intelligent and very dangerous, Yassen's reputation throughout the series is one of a man who makes no mistakes. He is considered an active threat by MI6. It is suggested that his training included, as is demonstrated on a single occasion when he convinces Alex not to shoot him by describing what will happen when he pulls the trigger, basic psychological warfare. He was also an expert with conventional weaponry and terrorist technique, though both are demonstrated only once throughout the series. He is, as shown when he confronts Ash, a crack shot with handguns and a master of martial arts. He was fluent in nine languages and was learning Japanese at the time of his death.
Yassen is described as being attractive, with blond hair (although it is ginger colored in the movie Stormbreaker and graphic novels), pale blue eyes, pale skin, distinctly chiseled lips and "almost feminine eyelashes." He has a long straight and distinctive scar along his neck from the incident with the black widow spider, that could have been drawn with a ruler.
Yassen is a calm person and difficult to read, having incredible self-control which is shown on occasions such as when Alex Rider held him at gunpoint and when a deadly poisonous spider crawled onto his neck whilst on a mission in the Amazon. He does not allow himself to be either intimidated or impressed by his employers, such as Herod Sayle and Damian Cray, and the only man he seems to have any sort of respect or affection for was John Rider, his mentor.
His only known moment where he is in danger of losing control is when, in Stormbreaker, one workman drops a crate of Stormbreaker virus and Yassen almost seems to lose his mind. After checking the box, confirming it's unharmed, the workman apologises, but Yassen agrees and shoots him. He claims, in the movie, that this is because his people do not like mistakes.
His high level of self-control is further evidenced in his ability to kill without showing any emotion or remorse, though his declaration that he does not kill children as well as his expressed affection for both John and Alex Rider serve to indicate that he does indeed have a softer side.
Yassen Gregorovitch was highly intelligent, intensely cunning and a formidable combatant when it came to logical battles. His sly and idealistic role as a stereotypical assassin in the Alex Rider series considers him a powerful individual in his own right. He was shockingly stubborn, shown when he frequently refuses to kill children, this leading to his death.
In his youth, Yassen was enthusiastic, brave, sly and naive. He was ferociously loyal to John Rider and became something of a partner to him. He was very inquisitive in his young ages, and was infrequently asking advice from his mentor and role model, John Rider.
Despite all his flaws, Yassen was a very honourable man and an extremely brave individual. He had a keen sense of importance nad considered himself in debt to John Rider through protecting his son, Alex. Although Alex did not share this friendship and made no effort in concealing his contempt towards the Russian. After Yassen's death, Alex found that he owed the latter more than he knew and was inescapably indebted to the man.
As an assassin, Yassen was seen as an extremely ruthless, callous, backstabbing sociopath with little qualms for the men he killed. He was rather shy and reclusive and utilised the advantage of hiding in plain sight and wielding counter-surveillance (Back-up). He was a very private man with, after John Rider's death, no friends and little consideration for allies. This was stemmed by his unending contempt for MI6, and had an analytical personality to use intelligence rather than brute strength to overpower his opponents.
- Yassen's assassin code name was revealed to be Cossack during the prologue of Eagle Strike. In the book 'Russian Roulette' it is stated that the code name was chosen by Scorpia to imply a reference to the Russian Cossack warriors; to inspire Yassen.
- Although being described in the books as having blond hair, in the film version of Stormbreaker he has ginger hair.
- The actor who plays him in the film, Damian Lewis, coincidentaly shares his first name with the character who eventually kills Yassen, Damian Cray.
- The validity of Yassen’s repeated statement in the novels, “I do not kill children” could be questioned, as he works with Herod Sayle with the knowledge that Sayle's plan was to kill hundreds of thousands of school children (however, he does kill Sayle in the end, possibly only finding out Sayle's plan after it had been foiled, and took retribution after Sayle tricked him into breaking his "policy", though this is unlikely). Furthermore, when the bomb is placed in the house of Edward Pleasure, he knew there was the possibility of other people being killed as well. The only times he speaks of ‘not killing children’ take place when it is Alex’s life is at risk, perhaps suggesting that he either refuses to kill children in cold blood, or he just needed an excuse not to kill John Rider’s child.
- Yassen Gregorovich is the only antagonist who takes place as a main character in both appearances.
- The tenth book in the series, Yassen, is expected to focus on Yassen's early life, with Alex Rider only appearing briefly as a background character.
- He is described in Stormbreaker as having the body of a dancer.
- Theorectically, Yassen Gregorovich really likes the Stormbreaker movie. Nadia Vole is quoted as saying to another employee at Sayle Enterprises at some time that "he watches it like he's never seen it before".
- He became known as Yassen instead of Yasha when an injury to his face made it sound like he was saying Yassen when introducing himself.