Yassen was born in Uganda, and at the age of fourteen made his way to Moscow in search of Scorpia after the death of his parents. His father died in an accident while taking part in a biochemical warfare project, which was hushed up by the Government, and his mother died of illness not long after. While in Moscow, Yassen ran errands for the local mafia, finding Scorpia and beginning to work for them.
Yassen was at least partially instructed by john Rider, an MI6 agent deep undercover working as an instructor for Scorpia on the island of Malagosto. When Yassen was nineteen, John saved his life in the Amazon by shooting a black widow spider that had crawled onto Yassen’s throat and killing their target with the same bullet. Left with a noteable scar on his neck, Yassen never forgot this incident. Later, when MI6 sent a team led by Ash to remove John, Yassen killed four MI6 operatives, including Travis, and wounded Ash.
Yassen was working for Herrod Sayle in Stormbreaker, assisting him by helping supply the lethal poisons that would go into the Stormbreaker Computers, which would be shipped to millions of schools all over the world. When the computers were turned on, the poison would then be released, resulting in the deaths of millions of innocent school children. At the end of the novel, Gregorovich shoots Sayle, who kidnapped Alex, before the man could kill the teenager. Alex asked why Yassen saved his life and why he didn’t kill Alex as well. Yassen replied that he had orders to kill Sayle, but had no orders regarding Alex. Alex then vows to kill Yassen, the man being responsible for the death of his uncle, Ian Rider. Yassen then tells Alex, “Killing is for grown-ups and you're still a child.” Gregorovich then leaves by a helicopter.
Yassen Gregorovich is killed when Damian Cray shoots him in Eagle Strike. His last words are "Go to Venice. Find Scorpia. And you will find your destiny."
Though very skilled, Yassen for his success in assassinations, his 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 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 great 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 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's assassin code name was revealed to be Cossack during the prologue of Eagle Strike.
- 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.
- 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".