Jacknife was a former runner and an antagonist.
Jacknife was an ex-Runner and the apparent mastermind of the events of Mirror's Edge, although he admits there are even "big[ger] fish" than himself. He was one of the primary antagonists of the game.
Jacknife is first mentioned on the final cutscene of the "Flight" chapter. Faith goes to see him, believing he might know something about Project Icarus, which she knew about because of a piece of paper taken from Robert Pope's office.
Meet with Faith
Faith then goes after him, finding him atop a building near the Old Runner Training Grounds, where he gives a cheery "Hello Faithy" and runs away, with Faith in hot pursuit. Faith manages to catch up with him after an elevator ride, where Jacknife tries to swing off a construction pole not knowing it was loose. He swings on it before losing grip, which makes him fall a fair distance to the ground and in turn is knocked unconscious. Faith uses another path to catch up with him, when he wakes up.
Being interrogated by Faith about Icarus and Pope's murder, he points to a big orange sign with the initials TB, and suggests Faith goes after him, but before leaving, he says cryptically that the "people always want to swim in the bigger pond.... they never see the bigger fish." Faith does not leave for Ropeburn immediately, instead she goes to see Miller.
After getting what she needed from Ropeburn, Jacknife is cornered by Faith on road level, where she once again questions him, this time about the masked sniper which shot both Pope and Ropeburn dead, as she thought he wanted him dead, though he negated the accusations, saying that he would just kill Faith himself if he wanted. She let him go, and then Jacknife told her that "you [Faith] need to be careful who you hang around with."
With Mercury now dead and receiving the assistance of Miller, Faith works her way up to the Shard's lobby, and there she meets, to her surprise, Jacknife holding a handcuffed Kate Connors hostage. Once again interrogated by Faith who's behind the plot, he only says "Let's say I know who the bigger fish are.", which Faith quickly deduces to be Callaghan.
Changing the subject, she asks why the Runners are targets. He replies (slowly bringing Kate to the helicopter stationed behind them) that without Pope and/or the Runners, the old city system will cease to exist, and the ones who do not accept the new system will be "cleaned".
Kate tries to push him away, but he punches her inside the helicopter, jumping in soon after. He signals the pilot to lift off, and signals Pirandello Kruger soldiers behind Faith Connors, saying: "You cannot live on the Edge all your life, Faith! Sooner of later, you'll have to jump!" Ironically, with Runner Vision, a handlebar on the aircraft turns red as it was lifting off, showing that the player must use Parkour.
The helicopter with Jacknife stops at the edge of the Shard, when he starts firing. Faith runs after the helicopter and swings at a bar whilst kicking Jacknife off the opposite end. While falling to his death, he managed to break the Helicopter's engine in his panicked firing, sending it spiraling out of control.