Grayson is tall and thin with dark hair. His addiction to red sand gives him red, bloodshot eyes and luminous teeth, common to all addicts. He could be described as xenophobic, due to his loyalty to human-supremacist Cerberus, but he genuinely believes in Cerberus' ideals.
Mass Effect: Ascension
The Cerberus Operative
In service to Cerberus, Grayson undertook many horrific missions. He made his bones by killing Keo, Claude Menneau's bodyguard. Most notably, Grayson sabotaged starships over colony worlds to release dust-form element zero. The worst of these was over Yandoa, which caused unknown numbers of birth defects along with thirty-seven biotic children, including Gillian herself.
Grayson was given Gillian, as a mission, by the Illusive Man when she was just a baby and told to raise her as his own daughter. His dossier would say that his wife died during child birth and he never remarried. According to the Illusive Man, Gillian was intended to play a key role in humanity's future, even to be a 'savior', and that Cerberus would help to nurture her potential.
Naturally, Grayson loved his daughter very much, even though they were not biologically related, and missed her greatly when she went to the Ascension Project. The pain of being separated from her made him more dependent on red sand until he was visibly addicted.
As Gillian's father, Grayson is able to pass on orders and special medicines to the undercover Cerberus operative at the Ascension Project, Dr. Jiro Toshiwa. When the medicine causes a seizure and Jiro's cover is blown, Grayson is ordered to remove Gillian and take her to a Cerberus facility, but he is blocked by Kahlee Sanders. Unaware of his true identity, Kahlee and Hendel Mitra travels with Grayson and Gillian to Omega in the Terminus Systems. Grayson secretly intended to meet with his old partner Pel, but Pel double-crosses him.
The four of them are held prisoner in an Omega warehouse, awaiting sale to the Collectors, but Lemm'Shal nar Tesleya launches a rescue attempt. Kahlee tells Lemm to leave Grayson behind; in the confusion, Grayson is able to break out and kill Pel in revenge. He also finds the sick quarian Hilo'Jaa vas Idenna and manages to recover access codes to the Migrant Fleet from him.
Attacking the Migrant Fleet
Believing the quarians took Gillian, and becoming more wary of Cerberus' methods, Grayson pleads with the Illusive Man to make him head of the retrieval team sent to the Migrant Fleet. The Illusive Man finally agrees and Grayson, along with the traitor Golo, leads a small squad of Cerberus commandos aboard the Idenna, using the scout ship Cyniad as a decoy. He plans to abandon the Cyniad and steal another shuttle to escape; the Cyniad is rigged with explosives to scuttle the Idenna in the attack.
Grayson finds Gillian returning to the shuttle where she was quarantined and stuns her, hoping to escape quickly, but Kahlee Sanders catches up with him trying to escape. As she is beaten viciously by Golo, Kahlee pleads with Grayson to see the truth - Cerberus is using his daughter, not turning her into a savior. All they care about is her biotic power and how they can use it as a weapon. Grayson finally makes his choice, shoots Golo in the head, and warns Kahlee that the Cyniad is trapped.
After the incursion on the Migrant Fleet, the quarians spends several days deliberating what to do with Grayson for the crimes he has committed against them. While Gillian prepares to leave on the Idenna's five-year scouting mission, custody of Grayson is given to Kahlee Sanders. She intends to turn him over to the Alliance for sentencing, and she and Lemm begins the journey to Cuervo with their prisoner. However, as Grayson tells her, Cerberus has eyes and ears across the Alliance, and taking him to Cuervo will effectively be a death sentence. Grayson manages to get hold of Kahlee's Ascension Project-issued stunner and knocks both her and Lemm unconscious. They wake up unharmed on Daleon, but Grayson has vanished.
Grayson later sends a message to the Illusive Man to tell him he is leaving Cerberus forever. He also ensures Kahlee Sanders' safety by claiming that if the Illusive Man ever takes action against her, Grayson will tell the Alliance all he knows.
Mass Effect: Retribution
Three years after his departure from Cerberus, Grayson has covered his tracks, but is very much aware that Cerberus is still relentlessly looking for him. He has overcome his addiction to red sand and started a new life on Omega under the alias "Paul Johnson". He maintains contact with Kahlee Sanders but never reveals his location or his activities. He chose to stay on Omega, because that is the part of the galaxy that the Illusive Man never got a proper foothold on. Despite his lingering xenophobia, he starts working for Aria T'Loak as an enforcer and even gets into a relationship with her daughter, Liselle.
Under Aria's orders, he along with Liselle and three other mercenaries attack a Talons warehouse and steal their shipment of red sand. The Talons are a turian gang who have become more prominent after a recent shakeup within Omega's underworld. Grayson experiences a crisis of conscience, as he does not want to give the red sand to Aria, and is fearful of relapsing into his old addictions. However, sensing that carrying bags of red sand through hostile territory is dangerous, he orders his squad to stash the packages in his apartment. After his men leave, he sends a message to Kahlee, maintaining his contact with her.
Grayson struggles with himself thinking about the packages of red sand; one part of him craved for a fix, the other, more dominant part, reminds him of his promise to himself to be a better man. Eventually, he overcomes his temptation and cleans himself up before heading to Afterlife Club. After meeting with Aria, he meets with Liselle, who offers to come to his apartment, but he turns her down due to his conflicted state of mind after talking to Kahlee. Later, after retiring to his apartment, he calls Liselle over, trying to forget the packages of red sand and hence "replacing one addiction with another".
While he is with Liselle, he is found by Cerberus operative Kai Leng, who reports his whereabouts to the Illusive Man. Kai Leng attacks Grayson's apartment with several other operatives. Grayson and Liselle put up a desperate fight but are overpowered, but not before Grayson sending a message to Kahlee saying that Cerberus has found him, and wiping his system files. Grayson is then brought to a Cerberus research facility.
Grayson is implanted with Reaper technology as part of an experiment to study Reaper enhancements. The Illusive Man himself is present at the time of Grayson's awakening and tells him that he is sacrificing for the good of humanity. Grayson could do nothing but scream silently as the Reaper nanides entered his body and latch on to his brain, transforming him into a Reaper controlled entity. Grayson tries with every fiber of his will to combat the Reapers' influence, but his own efforts are slowly being overshadowed by the Reapers' indoctrination. Despite this, Grayson puts up a stiff resistance and Cerberus had to use red sand to wear his will down. After several days, Grayson's body is seen to have undergone radical changes; his flesh becoming stretched and semi-translucent, the Reaper technology clearly visible under his skin, his senses, strength, speed heightening to unnaturally high levels and also developing considerable biotic powers. Despite this, Grayson continued to resist the Reapers' control.
When the station comes under attack from turian soldiers, Grayson is discovered and brought onto an evacuation shuttle. By this point, the Reapers have taken almost full control of his body and Grayson could do nothing as his body slaughters the turian soldiers in mid-FTL flight and takes control of the ship. The Reapers are able to force Grayson to send a distress call to Kahlee, drawing on his feelings for her. The Reapers then puts Grayson to sleep, needing their subject to be functional and ready. When Grayson comes to, he receives a return message from Kahlee asking him to come to Omega. Despite his resistance, Grayson is forced to head to Omega to find Kahlee.
In reality, Aria forced Kahlee, whom she has captured, to send the message so her mercs could ambush and eliminate Grayson as part of a deal with Cerberus. When Grayson arrives at the meeting point, he is ambushed, but owing to his enhancements, he tears through the mercs and flees to the spaceport. On the way, the Reapers delve into his mind and find out everything related to the Ascension Project. They then make Grayson board a ship and head to Grissom Academy. On the way, the Reapers have him create an advanced virus software to disable the Academy's communications.
Grayson is able to deceive the Academy authorities into boarding the station without confirmation of his identity. Once aboard, he kills the guards and heads for the Ascension Wing. After killing the scientists at the primary research lab, the Reapers used Grayson to hack into the archives and transmit all information to them. During the process, he is attacked by Anderson, who arrived on the station with Kahlee and Kai Leng. Due to his continuous activity from Omega, Grayson's body starts failing and the Reapers realize they cannot keep their avatar functioning for long. Grayson's abilities weaken and he is forced to fight Anderson from cover. When Kahlee arrives on the scene, Grayson biotically pushes both of them and abducts Kahlee once Anderson is disoriented.
Some part of his original self still manifests itself as the Reapers have Grayson explain to Kahlee their views. He still struggles to retain control, but Kahlee starts to fall under the Reapers' spell until Nick Donahue attacks them with a biotic wave that sends them sprawling. Grayson however quickly recovers and shoots Nick and is about to kill Leng when Anderson arrives and takes him down with a shotgun. His body spent, the Reapers release their control at last, he smiles up at Kahlee who comes over to him, and then Kai Leng puts two bullets into his skull.
Mass Effect: Deception
Note: This section features material from the first edition of Mass Effect: Deception, which BioWare has acknowledged contains discrepancies with other Mass Effect lore. Once revised editions of the book are released, this section will be updated.
At the time of Mass Effect: Deception, David Anderson and Kahlee Sanders bring Paul Grayson's body with them to the Citadel to show to the Citadel Council as evidence of Reaper involvement in the attack on Grissom Academy. The Council remains unconvinced, however, and believes that, as alien and disturbing as Grayson's mutations are, an organization like Cerberus could be just as capable of producing such technology. The fact that Grayson was once a member of Cerberus is further proof to the Council that only Cerberus was involved, not the Reapers. After learning of Anderson and Sanders' presentation, the Illusive Man orders Kai Leng to retrieve Grayson's body.
Mass Effect 3
During the assault on Cerberus's headquarters, Cronos Station, Shepard finds video logs detailing Grayson's fate. The Reaper implants within him proved useful in adapting the technology for Cerberus' forces.
He is also mentioned by Admiral Anderson when Shepard asks him about Kai Leng.