Lancel Lannister
Tywin's nephew and King Robert's squire, an eager, pliable young man forever trying to be of use to someone more powerful.
The fuller tale
Lancel begins as the court's least considered person: Kevan's eldest son, a pretty, eager boy squiring for King Robert, whose duties consist mostly of fetching wine and absorbing abuse. It is the wine that makes him historic. On the boar hunt that kills the king, Lancel keeps the skin full at his queen's instruction — strongwine, doubled — and thereby becomes the smallest hand on the largest lever in the realm: a squire's wineskin ends the reign of Robert Baratheon and starts the War of the Five Kings, and the boy pouring it barely understands what he is part of.
Cersei, who wastes nothing, collects him. With Jaime a prisoner of the Starks she takes her cousin — young, golden, and pathetically willing — into her bed and her service, and Lancel's brief career as a knight is spent running her errands and keeping her secrets, both of which Tyrion pries out of him with ease and turns to blackmail; for a stretch of the second season the wretched young man is a double agent for both siblings at once, and knows it. The Blackwater nearly kills him — an arrow, in the show; in the books a far graver wound taken fighting on the bridge of ships — and his convalescence becomes a conversion. Pain, guilt, and the memory of everything he did and poured accumulate into the one thing no Lannister is supposed to acquire: a conscience.
He returns transformed — gaunt, shorn, barefoot, a sworn brother of the Faith Militant who calls himself a new man. The family is appalled; Kevan grieves the son he cannot reach; and Cersei discovers that her discarded instrument has become her prosecutor, for it is Lancel's confession — the affair, Robert's wine, all of it — that arms the Faith with the charges that put a queen in a cell. There is a hard justice in it that even the show's compressed telling preserves: of all the people Cersei used and threw away, the one who brings her down is the one nobody, least of all Cersei, ever bothered to fear.
The page and the screen
Book Lancel's conversion runs deeper and stranger. Married off to Amerei Frey and granted a lordship at Darry as a war reward, he abandons wife, lands and inheritance entirely to join the Warrior's Sons — the Faith's revived knightly order — over his father's stunned objections, and his confession of sins to Jaime, delivered on his knees in a sept, is one of the books' quietly devastating scenes: Jaime, hearing his sister's adulteries recited by yet another man, begins his own turn away from her. The show trims the marriage and the order, keeps the informer role, and adds the finale errand the books never reached. As of the published novels Lancel is alive, praying somewhere in a city that has no idea what is coming.
Where they stand, season by season
Their fate — if death has found them, it is written here. Open at your peril.
The show: Died when the sept burned (S6·E10)
The cousin who shared Cersei's bed and then found the Faith followed a child into the tunnels beneath the Great Sept, and crawled with a cut spine toward the candles burning in a lake of wildfire. He was an arm's length short.
He dies in the show as the Faith's last errand-runner. Sent to fetch Cersei to her trial at the Great Sept, he follows one of Qyburn's little birds into the tunnels beneath it and finds the cache: casks of wildfire in a black lake, candles burning down toward it. The child puts a knife through his spine, and Lancel — legs dead, arms working — drags himself across the stone toward the guttering candles, an arm's length short when the flame meets the green. The explosion that vaporizes the sept, the High Sparrow, the Tyrells and his own father begins over his outstretched hand. In the published books he is alive, a Warrior's Son in a standing sept, his story open — the novels' Lancel has confessed his sins but not yet paid anyone else's.