Karliah is a Dunmer member of the Nightingales. She’s a former member of Skyrim’s Thieves Guild but has spent the last twenty-five years in exile after allegedly murdering the former Guildmaster, her lover, Gallus. She eventually becomes a quest giver and temporary companion to the Dragonborn after advancing in the Thieves’ Guild questline.
According to the in-game book The Nightingales Vol. 2, Karliah’s maternal grandparents were Queen Barenziah and a Nightingale thief named Dreyven Indoril. They had a daughter, Dralsi, who was taught the ways of the Nightingales by her father. Dralsi eventually got married and had a child she named Karliah.
Like Dralsi’s father did for her, Dralsi taught Karliah the art of thievery and how to survive in Skyrim living as a rogue. Dralsi did not, however, induct Karliah into the Nightingales before her death – this was done by Gallus Desidenius, an acquaintance of Dralsi and later Karliah’s lover.