"I don't blame him for being cautious. Hell, I'd be in a jail cell, or worse, if I hadn't helped track you down in New York. They don't fully trust me, probably never will." - Karen Walker, United as One
Special Agent Karen Walker is an FBI agent and former MogPro supporter, who was tasked in capturing the Garde. However, after what happened to her partner, Special Agent Purdy, she joins the Garde in exposing Setrákus Ra and the Mogadorians. She is currently completing an assignment to assassinate all surviving MogPro supporters.
Agent Walker's most notable feature is her red hair. Though John Smith notices her developing streaks of silver in The Revenge of Seven. John could also tell she went under extreme FBI training by the shape of her body.
Walker is known as the woman in charge. She does not let much else of her actual personality show. She is mostly strictly business. However, as the series progresses she begins to open up more, as she jokes about hormonal teenage problems when Sam Goode and Number Nine talk about Number Six and when John stops their van to call his girlfriend Sarah Hart. Walker also shows concern for John's safety in The Fate of Ten, and appears amused when John uses his powers against military forces to free his Mog ally Adamus Sutekh in United as One.
Walker and Agent Purdy track down Four and Nine and ambush them in an abandoned house in West Virginia. They lock the Garde in Plexiglas cages in the back of a van, however John and Nine escape and wreck the van, with the assistance of their Chimæra Bernie Kosar. The agents are heavily injured, unable to stop John from trying to get information from Walker, who falls unconscious from her wounds. He goes out to help Nine and returns to the van to find Walker missing.
Later she is present at Dulce, recovering from her wounds sustained by John and Nine. Walker briefly talks to Six, explaining to her that they are working with the Mogadorians in exchange for weapons, iridium, and knowledge of other worlds. However, Walker gets captured by the Garde but then presumably escapes. Although never stated or shown in The Rise of Nine, Walker witnesses her Agent Purdy have a heart attack and partially disintegrate. This causes her to question her faith in MogPro and is the catalyst to her future betrayal.
Walker and other FBI agents arrive at Ashwood Estates, waiting for a chance to strike before seeing Number Four take it over. Trying to make contact with the Garde, she attempts to convince them that they mean to ally with them. But the Garde are suspicious. She reveals to them that the Mogs came to Earth 10 years ago and, in exchange for advanced technology, medical enhancements, etc., the Mogs were allowed to use their resources to track down the Garde. Also, she reveals that about 15% of the USA's military, CIA, FBI, etc. are MogPro, along with some foreign officials and the Secretary of Defense and the Vice President (the US president is kept in the dark).
She and a couple of her men travel to New York with John, Sam and Nine to intercept the Secretary of Defense, Senator Sanderson, only to find him betrayed by the Mogadorians as his enhancements turn on him. After John heals the Senator, Walker takes the group to where Setrákus Ra makes his presentation to Earth. When Ra's true form is revealed, her and her group try to shoot him down, but Four stops them with intentions of saving Ella from the charm. She is later lost in the New York crowd while John and Sam attempt to rescues civilians.
Walker is at a military camp in New York and is searching for Number Four (John Smith). When she finds him she informs John that the president will not openly support the Garde if John does not figure out how to take down the Mogadorian warships. Later a group of soldiers come with one having the number five carved into his chest. Agent Walker takes a team of FBI agents with her and anger Five by pointing their guns at him. When Five reverts to his regular form and is healed John asks Walker to put him in a prison where he can't touch anything hard, to which she agrees.
Agent Walker then shows John a footage of some thing falling out of a Mogadorian ship, which is revealed to be a giant Mogadorian creation. Walker and her agents shoot at it, to no avail. She is confused when the Garde are pulled into a vision; for what seemed to be only a second. Daniela Morales then defeats the Mogadorian monster with her new Legacy, turning it into stone. Agent Walker speaks to the President, whose daughter had experienced the Loric vision. Sarah Hart then calls John, who tells Walker to put the President on hold.