Seymour Birkhoff is a Division agent recruited when he was found trying to hack into the Pentagon. Division recruited him for his technological prowess, and designated him for tech duty. He is a stereotypical "nerd", with a jocular attitude and a high opinion of his skills.
Birkoff was Recruited at the same time as Nikita. As a college student, he was caught hacking into the Pentagon's system and was recognized by Division for it. When he actually came to Division, it was Nikita, Birkoff and Michael that were usually put together for missions once Nikita progressed beyond recruit status. Michael was also Nikita's trainer.
In the pilot episode, Birkhoff is kidnapped by Nikita, who is trying to find information on DIvision. She "pistol whips [his] face", yet allows him to live after this. Michael and other Division agents find him taped to a piece of playground equipment, much to Michael's amusement.
Birkhoff is thought to be Nikita's mole in Kill Jill when he makes the snide comment that the "only person who could hack into Division is me." This rouses the suspicions of Percy and Michael, whom send him to talk to Amanda. Amanda sends Birkhoff to Medical for a brain scan (as well as a full body cavity search), and Birkhoff is found to have a "tiny wireless transmitter" embedded in his second biscupid that was placed there by NIkita when she kidnapped him.
He is also implied to have a crush on Nikita, often calling her "Nikki" and telling Michael that he misses her, too. This is often joked about, especially towards the end of the series when Michael and Nikita eventually become a couple.
In terms of personality, Birkhoff stands out amongst Division agents and recruits alike. He has a stereotypical "nerd" sense of humor and a jocular attitude towards even some of the more serious sides of Division. He is shown to enjoy video games in his spare time and has a passion for the good technology that Division provides for him.