Agent 355 serves as Yorick Brown's bodyguard. Her name is pronounced "three fifty-five", not "three-five-five". As of yet her true name has not been revealed.
It's believed that she may have inadvertently started the plague that killed all the male mammals in the world. She was on a mission in the nation of Jordan to save Dr. Frozan Hamad and an amulet in her possession. Dr. Hamad told her that it was a family heirloom. She mentioned that her father told her that a catastrophe likened to the Trojan War would happen if the amulet was removed from Jordan. After Dr. Hamad was killed by people after the amulet, Agent 355 took the Amulet of Helene. As soon as the Amulet entered Saudi Arabian airspace, the plague hit.
Bodyguard to Yorick Brown; Her parents and sister were killed in a car accident when she was young. While fleeing a foster home after a fight with to racist boys she is recruited by the Culper Ring, a US government agency that can trace its origins back to American Revolutionary War. She is skilled with firearms, but her trademark weapon is a collapsible baton. She is reluctant to kill, but becomes more cold-blooded after hearing of the death of her childhood friend Agent 711. She also appears to be able to quickly remember minute details.
355 also possess the odd quirk of knitting balancing out her tough exterior.
Her first code name was 86, then changed it to 355 after killing her former mentor (355) and thereby preventing an assassination attempt on the President.
Agent 355 protects Yorick as they travel with Dr. Alison Mann. They must reach Dr. Mann's back up research facility in Los Angeles. Along the way, they face off against various groups endangering their mission. Once in Los Angeles, Agent 355 is confronted by a splinter group of the Culper Ring called the Setauket Ring. The Setauket is now headed by Agent 355's former associate Anna. They briefly escape after stealing Yorick's magic ring which he believes is the reason he survived the plague. Once back at the Mann's back up lab. Agent 355 declares that they must evacuate do to the threat of the Setauket ring. However, at this point, Yorick begins showing signs that he may have finally succumbed to the plague. Agent 355 finally meets up with the Setauket ring in order to exchange the Amulet of Helene for Yorick's ring.
At this point, Hero, Yorick's sister and former Amazon, finds Agent 355 and the Setauket Ring and tells of finding Agent 711's corpse confirming that the Setauket killed 355's friend. An uneasy alliance forms between Hero and Agent 355. Eventually, 355 kills the two members to face off against Anna. 355 breaks Anna's neck and leaves her to suffer and call off the Setauket Ring rather than killing her.
Later, a ninja kidnaps Ampersand, Yorick's monkey and the newly discovered key to save the human race from extinction. 355 and Yorick follow the ninja to a bridge in which they fight. Impressed with 355's skills, the ninja (later to be discovered to be Toyota) gains the upper hand and slices Agent 355's arm, and then escapes with Ampersand.
The group then follows Ampersand through a GPS-tracking device to Japan and then China after some mishaps along the way including a brief hook up between 355 and Dr. Mann.
In China, the group is apprehended by Dr. Matsumori, Dr. Mann's father and another living male post-plague. It is discovered that he may have caused the plague due to his experimentation with cloning. As this is discovered, Rose (the Australian Navy personnel now traveling with the group) and Agent 355 are suspended in a lower floor at the mercy of Toyota. Using some of the skills Yorick taught her in lock-picking, 355 frees herself from her cuffs. She then rushes Toyota through a window to land on a lower floor.
Toyota and 355 fight with Naginata on the rooftop. Toyota splinters the end of her Naginata at the blunt end when ramming it into 355's chin. Agent 355 takes the splinter end and stabs it into Toyota's ear and killing her but at the expense of the Naginata blade stabbing her through the stomach.
After healing, the core group breaks up and Agent 355 travels with Yorick to get to Paris through various train lines. Once there, Agent 355 gives Yorick the scarf she had been knitting over the last 5 years as a present. While presenting his thanks, 355 disappears and suddenly Yorick's girlfriend, Beth, who he had been looking for for 5 years appears. Overjoyed, Yorick also realizes that 355 had arranged the meet-up.
Later, Agent 355 is seen at a dress store in Paris where she trades the storekeeper her gun for a black, sleek dress. She returns to her room which is later found by Yorick returning to her after a fight he had with his girlfriend. He admits that he feels that though they have been companions, he has also realized that he has loved her during their entire journey. This is confirmed with his realization during his stay at Agent 711's when 355 appeared to him to stop him from being so reckless and suicidal instead of Beth. They talk about their future and finally, while hugging, 355 whispers her true name to Yorick. He laughs but finds it to be perfect for her. Then, after this brief moment of clarity, 355 is sniped through the forehead from a nearby rooftop.
Yorick falls to the ground with his hands soaked in 355's blood. A sleeping gas can is thrown through the window, and Alter, the Israeli Sergeant comes through the building only to find Yorick with his gas mask on with the collapsible baton in his hand. He gains the advantage and nearly kills Alter but realizes that she has had the same suicide pact the whole time. He brings her out to her small troop and leaves her. The rest of the group later finds Yorick crying at a fountain over the loss of his true love.