Kathryn O'Brien was a CIA agent until illegal operations were exposed, after capturing Frank Castle aka The Punisher led to a run in with mafia boss Nicky Cavella and his goons, which then turned into a bloodbath at a New York hotel. Serving 2 years in prison, she escaped and ran into Frank Castle again when she discovered that her ex-husband, Bill Rawlins, had been hired to assassinate the notorious vigilante.
She had worked within the CIA at Rawlins side in the past only to be betrayed and left for dead in Afghanistan at the mercy of terrorists, where she was taken prisoner and raped until she managed to get away.
Her main goal was ultimate revenge on all the low-lives with no value for human life that took advantage of her, which was why she found a kindred spirit in The Punisher, and someone who would jump her bones whenever it suited her.
O'Brien was written by Punisher MAX writer Garth Ennis and designed by artist Lewis Larosa. It's implied throughout the series that O'Brien is the same character as Kathryn McAllister from Ennis's series Hitman. In Up is Down, Black is White, it's mentioned that O'Brien used to go by the name McAllister and in Man of Stone, she mentions an old boyfriend whose name was Tommy.
In the Beginning
Working for corrupt CIA boss Robert Bethell, Kathryn and her fellow agent Roth are employed to work with Linus Lieberman aka Microchip to apprehend Frank Castle, the Punisher. Bethell wants to use the Punisher to run operations in Afghanistan to protect his heroin business. However the mob have brought in disgraced mafia boss Nicky Cavella from Boston to take care of the Punisher once and for all and when all of the parties collide in an explosion of violence, Punisher is unleashed and springs into action. Upon staking out the Punisher, Kathryn is immediately intimidated yet attracted to Frank and when he is captured, she cannot help but let her feelings intensify for the old monster. When the CIA's base of operations is breached by the mafia, O'Brien is wounded but when the Punisher escapes by her, she cannot help but let her excitement get the better of her.
Up Is Down & Black Is White
After Frank has seen the other side of other battles, the one that got away - mobster Nicky Cavella - is back to prove himself. He's escaped the Punisher once and thinks he can finally bring the brute down. He begins by sending film footage of himself having dug up the graves of Frank's family and desecrating them with a smile on his face. Trying to convince the remaining mobs that it can be done and that he can be trusted after having been responsible for many deaths the last time around, Cavella employs rogue CIA agent William Rawlins to assassinate the Punisher but it's just not going to be that simple.
Rawlins' ex-wife, who he left for dead in Afghanistan has escaped from prison and is hot on the trail of where her story last left off. When O'Brien and her friend Roth come to the aid of the Punisher and in the same time incapacitate and kidnap Rawlins, so begins a mutual alliance and yet also the continuation of Kathryn's obsession with Frank Castle, in which they sleep together to pass the time. They torture Rawlins for evidence of corruption within the CIA and the US Military and in the end Frank returns to put an end to Nicky Cavella once and for all. But Rawlins has escaped and is on the loose once more.
Man of Stone
Kathryn O'Brien has returned to Afghanistan to get revenge on the people that imprisoned, raped and tortured her and meanwhile, Russia's most fierce military legend of the Cold War and an old enemy of Frank's - General Nikolai Alexander Zakharov, the Man of Stone - is back in the country and dealing with Rawlins. The only thing that will save Rawlins' life after he last double-crossed the General is an offer he can't refuse and that is the life of the Punisher. Upon being captured by Frank's old war buddy, SAS Major Yorkie Mitchell, Kathryn is reunited with Frank as he flies in to lock horns with the Man of Stone, but just as Frank and Kathryn prove a force to be reckoned with, so does Rawlins who double crosses Zakharov one last time and leaves him for dead.
In a shock twist of events, Kathryn steps on a landmine and dies in Frank's arms, but only some time after asking for a last request. Frank buries her and goes on to fulfil the request, hacking Rawlins to death with a machete in a public toilet.
Long Cold Dark
Long Cold Dark begins with the return of Barracuda, who has tracked him down after interrogating Yorkie Mitchell for information before killing him. He tells Frank that he is the father of a baby girl. Some time between the events of Black is White and Man of Stone, Kathryn bore Frank a daughter, but knowing that neither of them can take care of her, she gives the baby to her younger twin sister. Barracuda becomes aware of her and kidnaps her. Accepting the way things have to be, Frank makes sure his daughter will be secure, makes his peace and sets off down the long cold dark that has become of his life.