Sean 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 had deep feelings for Alex.
Sean appeared in "Game Change" as a newly recruited agent. He also met Alex in this episodes.
In "Sanctuary" Sean attacks 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 "Black Bridge" Sean was framed for the murder of Kendrick. Sean then turns himself in for questioning to by time. In "Broken Home", Sean is wounded in action with a bullet and dies later on 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 is very loyal to his mother, Madeline Pierce. He will "do whatever it takes" to protect her.
- 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.