Union of Allied Planets agent
Lawrence Dobson was an undercover Union of Allied Planets agent, with a distinct garlicky smell, in pursuit of Simon and River Tam. When he suspected to have found Simon on the planet Persephone, he joined him and the Shepherd Derrial Book as a passenger on board Serenity.
Soon after leaving the planet, he attempted to contact the Alliance to report his discovery. However, his transmission was cut by Wash
before he could get any more info out. Captain Malcolm Reynolds assumed that Simon was the Fed and confronted him in the cargo bay. However, Book pointed out that Dobson was the real Fed, as he was brandishing a gun at Simon. Dobson threatened to have Reynolds and his crew prosecuted for transporting a fugitive. Moreover, he wasn't convinced that Serenity was actually transporting medical supplies for the Alliance. After a moment of panic, he shot Kaylee and was knocked out by Book. He was locked up for a while, and when interrogated by Jayne, he attempted to bribe him for his freedom.
By the time the ship arrived at the moon Whitefall, Dobson escaped his quarters, beat up Shepherd Book and attempted again, unsuccessfully, to contact the Alliance. He revealed a more sadistic side of himself as he beat Book unnecessarily in a fit of anger. Up to this point Book had been protecting him from harm, mainly by Jayne. He then found River and tried to leave the ship while holding her at gunpoint. Malcolm Reynolds, returning from his mission, sized up the situation and without hesitation shot Dobson in the head. Mal and Jayne then tossed him from the ship, leaving him for dead.
Lawrence Dobson later returned, sporting an ocular implant where Mal had shot him in the eye. Obsessed with seeking revenge on Reynolds, for the Alliance declared him dead without investigation, Dobson joined the "Hands of Blue" in finding Serenity, its captain, and the Tam siblings. When the two finally met again, Mal shot the former agent first (in his good eye), and killed him for good.
Behind the scenes
Lawrence Dobson was portrayed by Carlos Jacott. In the "Serenity" episode, Dobson's initial clumsiness and common appearance contrasts him to the viewer (and the other characters) with the more suspicious-looking Simon Tam, to conceal Dobson's role as villain on the ship. This is an effect shared by other Joss Whedon characters portrayed by actor Carlos Jacott—the enslaver demon (and Moo Goo Gai Pan aficionado) Ken and pseudo-cannibal (and part-time DJ) Richard Straley—where he also switches from seeming innocent, clumsy and unimportant to being someone with more malicious intentions.
In the commentary for the pilot episode, Joss Whedon mentions that Dobson would have become a recurring villain had the show gone on. However, he was able to bring him back in the comic book series that bridged the series and the movie.