Sean Mason Pierce was an Oversight agent and a Navy Seal assigned by Oversight to keep an eye on Amanda. He was a love interest to Alex. In "Clawback", it was revealed that Sean's mother Madeline was in fact a member of Oversight. Sean was shown to have deep feelings for Alex.
Sean first appeared in "Game Change" as a new supervisor sent from Oversight, this is also the first time he and Alex interacted.
He managed to deceive Alex by giving her the watch of her father with a tracker in it. His mother later informed him that they might completely wipe out Division. When Alex joined Nikita again he used the tracker to find out their location.
In "Sanctuary" Sean attempts to kill Nikita, but he fails and is captured. After a conversation Sean agrees to lie to his mother to help her stop Clean Sweep and save lives within Division. But when they called Madeline, Guardians came to Oversight and captured all the members.
In "Clean Sweep" Sean along with Nikita's team, came to save Oversight. They met Percy, who ordered to kill everyone, except Madeline. After shooting Nikita rescued Sean's mother and they all agreed to work for stopping Percy, who is going to regain his power.
In "Doublecross", Sean joins Alex in rescuing her mother Katya, as well as locate and destroy the last black box. However, Semak had already moved Katya, though Sean decided to copy the files off of Ari's laptop. In scouring through its files, they both uncovered Cassandra as Gogol.
In Season 3, Alex and Sean's relationship becomes strained when Sean sees Alex isn't distancing herself from Division, and her continued service was endangering her life. He was also jealous of Owen/Sam and Alex's relationship. Sean briefly left her, not wanting to witness another person he loved die due to their connection to Division, and it resulted in Alex relapsing into drug use. Later, however, Sean returned to save her life, realizing how much he needs her. Amanda targets Sean due to his place as a member of Nikita's team. Efforts to clear Sean fail, forcing Division to fake his death. Sean was then forced to remain at Division, his sisters believing him dead and the public believing him a murderer and traitor.
In "Black Badge" Sean was framed for the murder of Kendrick. Sean then turns himself in for questioning to buy time. In "Broken Home", Sean, involved in the mutiny, is wounded in action with a bullet and dies later in Alex's arms from internal bleeding, before telling Alex to promise him she will get better.
Being a military man, Sean had a code of honor that makes him disagree with many of Division's policies. In "Looking Glass", when a Division agent Lisa was captured, Sean insisted that Amanda provide him operational support instead of abandoning her agent, yelling "You never leave a man behind!"
While working with Division, Sean has been trying to work on a path of right and wrong, such as when he tried convincing Oversight to spare Ryan Fletcher or Nikita to turn Jonathan Gaines over to the proper authorities.
However, despite being a military man and staying true to his mission of retrieving the Black Box from Nikita, Sean's choice on how he will do so seems questionable. In "London Calling" he tells his mother that he plans on seducing Alex as a means of luring out Nikita, since the two still have a connection.
Sean begins to develop feelings for Alex in Season 2. He later becomes jealous of Owen/Sam and Alex's relationship. He tries to get Alex to tell him about her butterfly tattoo and tries to break down her walls. He gets Alex to open up to him.
Sean is very loyal to his mother, Madeline Pierce. He will "do whatever it takes" to protect her.
Sean is a highly trained soldier. As an elite soldier, Sean is skilled in the use of firearms, ranging from sidearms to sniper rifles. Sean is also skilled in military tactics and fighting at close range using edged weapons and unarmed.
- He was the only agent of Division to be recruited during Ryan Fletcher's administration, albeit they did not have a choice since Amanda ruined his life by wrongfully incriminating him.
- He was also the final agent to have ever been recruited before Division was shutdown.