He is a brother of the Night's Watch who has gone north of the Wall on Lord Commander Jeor Mormont's Great Ranging. Karl led a mutiny at Craster's Keep in which he and his fellow mutineers ―mainly ex-criminals― killed Craster and Mormont, but he and his comrades were later tracked down and killed by a band of the Night's Watch led by Jon Snow.
Karl Tanner hails from Gin Alley in Flea Bottom, King's Landing, where he was a self-proclaimed "fookin' legend", making a name for himself as a highly-capable enforcer, assassin, and cutthroat. Karl was eventually sent to the Wall and became a ranger of the Night's Watch. He takes part in the Great Ranging.
His fighting style favored speed and using small, fast weapons like daggers to quickly stab and deliver killing blows. This style reflected his background in Flea Bottom where he fought in tight, confined spaces and had to use small, easily-concealed weapons to avoid being caught by the city guard.
Karl survives the White Walkers' attack on the Fist of the First Men, and regroups with the remaining survivors as the ranging attempts to pull back to the Wall.
When the Watch arrive at Craster's Keep seeking food and shelter, Karl is openly antagonistic towards Craster, accusing him of giving his pigs better food than them. Along with Rast, he shows an interest in Craster's daughter-wives.
Following the death and funeral of Bannen, as tensions between Craster and the brothers of the Watch run higher, Karl insults Craster - calling him "a gods-cursed, daughter-fucking wildling bastard" - after the wildling demands the Watch to leave his hall and threatens to kill anyone who insults him. Craster charges against Karl with an axe but the black brother easily kills him by quickly closing the distance between them, blocking Craster's attack by grabbing the wrist of his weapon hand, and stabbing him in the throat with a dagger. Karl's action triggers a mutiny that causes the death of Lord Commander Mormont and fight among the black brothers.
Grenn and Edd arrive at Castle Black after escaping from their captivity at Craster's Keep. They reveal that Karl is the ringleader of the mutineers, and that they are content with staying at the keep, eating Craster's food, and raping his wives and daughters.
Jon Snow then points out that they know about the small garrison at Castle Black and proposes that they should be killed so they won't disclose it to the wildlings.
At Craster's Keep, Karl drinks wine from the skull of Jeor Mormont, drunkenly bragging about himself and his skill as a hired killer back in King's Landing before ordering Rast to "feed the beast". One of Craster's former wives enters the Keep and presents Karl with Craster's last child, a son. The women call for it to be given to the gods, which Karl realizes must refer to the White Walkers. Realizing that this was how Craster protected himself from the White Walkers, Karl orders Rast to take the child out into the forest, leaving it behind. Soon after, Bran, Jojen, and Meera are captured and taken to Karl, who threatens to kill Meera and Jojen until Bran admits his true identity as Jon Snow's half-brother.
With his new captives in hand, Karl attempts to rape Meera, but is interrupted when Jon Snow and a handful of the Night's Watch storm Craster's Keep. Karl duels Jon and initially Jon gains the upper hand but Karl uses dirty tactics to reverse the odds and is about to kill Jon when one of Craster's daughter-wives stabs him in the shoulder. As Karl advances towards the girl in anger, Jon kills him by driving his sword through the back of Karl's head and out through his mouth.
Karl's body is later burned along with those of the remaining mutineers and slain brothers inside the Keep to ensure that they cannot come back as Wights.
A native of Flea Bottom, Karl openly challenged Craster's treatment of his Night's Watch guests during a stay at his keep. His insults set in motion an uprising that resulted in the deaths of the Lord Commander and Craster. Karl was killed by JonSnow and one of Craster's wives when Jon recaptured the keep.