Kal was originally part of a tribe that died in the cold. He claimed to have been their leader. Following their demise, he crossed the mountain range that divided his tribe and the Tribe of Gum. (PROSE: Doctor Who and an Unearthly Child) When encountering this tribe, their leader, Za, chose not to kill Kal and permitted him to eat with them and share their caves.
Whilst Za spent his time attempting to make fire, Kal brought meat to feed the tribe. This led to Horg and the other old men of the tribe to begin favouring him and considering making him leader. Whilst Kal could not make fire, he claimed to have often seen men make fire where he was from. (TV: An Unearthly Child)
Bid for leadership and death
Kal saw his chance to become leader when he observed the First Doctor emerge from the TARDIS and produce fire by striking a match on a rock and breathing smoke through a pipe. Kal captured the Doctor and took him to the tribe, where he hoped to force the stranger to tell him the secret.
The old woman helped the Doctor and his companions escape the Cave of Skulls into the forest. Kal killed her in anger and blamed Za, but, when the strangers returned with Hur and an injured Za, the Doctor proved it was Kal who had killed her. The tribe drove him into the darkness.
Kal was desperate to become leader of the Tribe of Gum and was willing to lie and murder to achieve his goal. He lied to Horg and the old men of the tribe to curry their favour and murdered the old woman to frame Za. (TV: An Unearthly Child)
Kal was a highly skilled hunter, more skilled than any other members of his adoptive tribe, which earned his acceptance into the tribe. This skill was in part owing to his patience and cunning, which helped him to always return home from a hunt with a kill. Kal was also more intelligent than the rest of the tribe, which helped fuel his curiosity, as well as his ambition. (PROSE: Doctor Who and an Unearthly Child)
Behind the scenes
Kal has the honour of being the first villain on Doctor Who. Though Kal himself does not appear until the second episode of An Unearthly Child, "The Cave of Skulls", his shadow appears falling over the landscape during the last few seconds of the first episode, also called "An Unearthly Child". In fact it is not the shadow of Kal's main actor, Jeremy Young, which is seen, but instead that of stand-in Leslie Bates.
In earlier drafts of An Unearthly Child, Kal is instead named Gum. This is where the working title of The Tribe of Gum originated from, though in the final draft, the Tribe is given no such name. Later stories would revive the Tribe of Gum, adding this back into Doctor Who continuity.