The Assistant

"'A man so dedicated to the cause that he carved lines into his own flesh to act as cover.'" The Assistant was a recurring character for the duration of series 1 of the British. A high ranking member of the Network, he was a mysterious figure never referred to directly. Despite being Letts' assistant, he was the one seen giving most of the orders within Corvadt.

Background
In the 1970s flashback episode, the Assistant is revealed to be an early member of the Network and Milner's right hand man, accompanying her everywhere on the job other than her personal meetings with Philip Carvel. He is the only character who refers to her verbally as "Letan." The Assistant has been in the know about Janus since the beginning of its development and expresses his disgust at Carvel's interest in selecting an exceptionally fit population to be exempted from sterilization, calling the idea "the Third fucking Reich."

He carves the Chinese character for "rabbit" into his chest in order to serve as a decoy Mr. Rabbit in case Milner is outed as the true mastermind. It is implied that this was Milner's idea, and upon seeing the fresh scars, she tells him that "[his] loyalty breaks [her] heart." The Assistant is present when Milner drowns her husband, Tom, as well as immediately after she discovers that Carvel put Janus in Jessica.

Series 1
The Assistant is first introduced as Conran Letts' assistant at Corvadt Ltd. during a meeting with Greg Ashby, head of the Health Department and Michael Dugdale. After Greg leaves the room to take a call, the Assistant suggests that Dugdale convince him to sign off on the Russian flu vaccine. The Assistant calls it Dugdale's "mission", referencing an earlier call in which a presumed Network agent told Dugdale to do an unstated "mission" for them or he would reveal that Dugdale got Anya pregnant.

The Assistant tells Letts that it is time to give the order for Arby to begin massacring Grant's school. When Arby tells them that he obtained the manuscript, they discuss Jessica Hyde's removing of several pages and he mentions the possibility that Arby had her but let her go. He later gives Letts the sample of the Russian flu vaccine and sends him to take it to a testing facility.

When Letts is captured, The Assistant speaks with Geoff but doesn't seem to care very much about the situation, instead focusing on how poorly Geoff has handled the situation with Dugdale and that he should have solved the problem by now. Geoff quickly reminds him that he is an MP and that he doesn't take orders from him. When Letts returns after Wilson released him, The Assistant tells Letts he has served his purpose and orders Geoff to kill him -mirroring the earlier scene- to which he complies.

In S01E06, Wilson Wilson and Jessica Hyde find the minutes of a Defense Department meeting of the Committee of Genetic Discovery. A footnote in the minutes includes a reference to "Letan," who they know to be Mr. Rabbit. Dugdale is able to identify the Assistant in a photograph of the people at the meeting, and Jessica sets out to kill him, believing him to be Mr. Rabbit.

Separately, Grant is held captive in an underground detention cell where the Assistant is forcing him to try and remember the last digits in the chemical sequence of Janus as written in the Utopia manuscript. Grant kills the Assistant by stabbing him in the neck in order to escape, and checks his torso for the Chinese character for "rabbit," finding it there. Grant reunites with Jessica soon after and indicates to her that he has killed the Assistant, though Grant has no idea who he is. Jessica then believes Mr. Rabbit has been taken care of. Later, Jessica realizes that Milner is the true Mr. Rabbit. When Jessica asks who exactly it was that Grant killed, Milner responds cryptically (see header quote.)