Lancel Lannister was the squire to King Robert Baratheon and ensured that Robert had plenty of wine during his final hunt. Following Robert's death he becomes the lover of Queen Regent Cersei Lannister, his first cousin, and is also elevated to knighthood.
Lancel Lannister is the son of Ser Kevan Lannister and the nephew of Lord Tywin Lannister, the head of House Lannister and Lord of Casterly Rock. He is the first cousin of Jaime, Cersei, and Tyrion Lannister. He has the familial blond hair and resembles his cousin Jaime. He is a squire to King Robert Baratheon.
King Robert has Lancel pour wine for him while discussing old battles with Ser Barristan Selmy. Robert mocks Lancel's alliterative name, asking if his father had a stutter. Robert becomes annoyed when Lancel runs out of wine and sends him for more. At the Hand's tourney, Lancel tries in vain to help Robert into his armor, tactfully suggesting that it was "made too small". Robert angrily replies that Lancel's mother was a "dumb whore with a fat arse". At this point Eddard enters the tent and bluntly states that Robert's armor doesn't fit, because he has grown too fat for it, to the king's amusement. Robert then sends Lancel out in search for the "breastplate stretcher" (a nonexistent device). Eddard seems to feel sorry for Lancel, while Robert hopes that he is sent all over camp looking for this thing. Eddard says he should stop torturing Lancel
Lancel accompanies Robert, Renly and Ser Barristan on a hunt, constantly giving Robert wine. He is present when Robert is mortally wounded by a wild boar. Varys insinuates that Lancel's readiness with the wine may have been deliberate in order to slow Robert's reactions.
While Jaime is being held captive in the Riverlands, Lancel enters into a sexual relationship with his cousin Cersei. The Queen receives news of the war. Lancel questions her about it, he finds it exciting and wants to know what she will do. Cersei is irritated and tells him to stop talking and get back into bed.
Lancel attends court in the Iron Throne room following the defeat of Stafford Lannister in the Battle of Oxcross. King Joffrey Baratheon aims an ornate crossbow at Sansa Stark and says that she is there to answer for Robb Stark’s treasons. She denies having any involvement in Robb’s actions, calling Robb a traitor. Joffrey instructs Lancel to tell her of Robb’s latest outrage. Lancel claims that Robb used sorcery and an army of wolves to kill thousands of men under the command of Stafford. He adds that the Northmen feasted on the flesh of the slain after the slaughter drawing cries of outrage and horror from the courtiers. Joffrey orders Ser Meryn Trant of the Kingsguard to beat Sansa but spare her face. Tyrion arrives and halts the torture of Sansa.
Tyrion has had Grand Maester Pycelle arrested for breaking his confidence and feeding information to Queen Regent Cersei Lannister. Cersei sends Lancel to Tyrion with a warrant for the release of Pycelle. He arrives late at night and Tyrion invites him for a drink and Lancel refuses. Tyrion wonders why Cersei did not come herself. Lancel says that he does not care and Tyrion continues to question him. Tyrion establishes that the order came directly from Cersei and then asks why Lancel waited to deliver it. Lancel says that he came immediately and Tyrion wonders what he was doing conferring with Cersei so late at night. Lancel improvises and says that Cersei often works through the night, and Tyrion retorts that she must be glad to have Lancel helping her at all hours. Tyrion notes the scent of lavender oil on his cousin and says that it is a favorite of Cersei. Lancel asserts his position as a knight, and Tyrion jokes that he is an anointed knight. Tyrion asks if Cersei had Lancel knighted before or after she took him into her bed. Lancel furiously calls that a lie, but when Tyrion threatens to tell Joffrey, he collapses into a chair, aghast. Lancel claims that it wasn't his fault, and Tyrion asks if she took him against his will. Lancel insists he was following Tywin’s orders to obey Cersei in all things, but Tyrion counters by asking him if Tywin ordered him to sleep with her as well. Tyrion points out the benefits Lancel has received and threatens to go straight to Joffrey at which Lancel falls to his knees and begs him for mercy and offers to leave the city. Tyrion says that Lancel must act as his spy to buy his silence. Tyrion asks Lancel to beg Cersei’s forgiveness and agrees to release Pycelle but refuses to reinstate him to the council.
Bronn stands guard as Tyrion meets with Lancel in a litter on the streets of King’s Landing. Lancel reveals that the Queen’s plans for defense of the city hinge on the use of Wildfire. Tyrion is disbelieving and threatens to reveal Lancel’s sexual relationship with Cersei to Jaime Lannister. Lancel offers more information to back his claim; Cersei has met with the Alchemists' Guild and they have already stored thousands of pots of the substance. He says that they are planning to launch it from the walls onto Stannis’s fleet and army. Tyrion sees that Lancel is telling the truth and asks when Cersei told him. Lancel says that he has heard Cersei talking with a pyromancer and has visited their guild. Tyrion forces Lancel to swear that his words are true but tires of toying with his cousin and dismisses him. Tyrion kicks him over as he ducks out of the litter and then instructs him to tell Bronn to kill him if anything happens to Tyrion. Lancel dutifully does as he is told. Bronn says that it would be his pleasure as Lancel jogs away. Tyrion uses the information to usurp Cersei's connection to the Alchemists' Guild.
Lancel is on the walls of King's Landing during the Battle of the Blackwater and fights relatively well during a sortie led by Sandor Clegane. However, Lancel is moderately injured when he is shot in the arm by an arrow. Lancel uses the opportunity to retreat from the walls and head back to Cersei to give her a report on the battle. Cersei then drunkenly orders him to go back to the walls and bring Joffrey back to the safety of the Red Keep. Shocked at these orders, Lancel returns to the walls and reluctantly repeats them to Joffrey and Tyrion. Joffrey uses this obvious excuse to make a cowardly retreat from the walls. His soldiers are stunned that their king has abandoned them. When Lancel returns to the Red Keep, he angrily tells her that her order to retrieve Joffrey is costing them the battle, as many of the Gold Cloaks lost heart and abandoned the fight at the sight of their retreating king. Lancel firmly tells her that he must return Joffrey to the walls if they have any hope of rallying their defense again. Cersei flippantly brushes him off, at which Lancel stands up to Cersei for the first time and demands that she listen to what he is saying. In response, Cersei punches the arrow wound in Lancel's arm, causing him to crumple to the ground in severe pain. Cersei storms out of the room as he lies screaming on the floor.
Lancel survives his injuries, but retreats from court life while he recovers.
When attempting to barter for Tyrion's life, Jaime reminds Tywin that should Tyrion be executed the Lannister legacy will ultimately fall to Lancel, as Ser Kevan Lannister's heir.
Lancel returns to King's Landing for the funeral of Tywin after recovering with his father Kevan. During his time in recovery, Lancel has become extremely pious and joined the Sparrows. At Tywin's wake, Lancel greets Cersei, his attire and attitude clearly an embarrassment to Kevan. He later talks with Cersei privately, and asks for her forgiveness for their "unnatural" relationship, and for serving Robert the wine that led to his death. Cersei brushes off Lancel's request, and his prayers for Tywin's soul.
At midday, Lancel and some his fellow Sparrows invade one of Littlefinger's brothels. They chase away the whores and grab the High Septon, who vehemently protests but Lancel quickly silences him labeling him both sinner and traitor. When Olyvar tries to intervene the Sparrows give him a violent reprimand. Lancel forces the High Septon to walk the streets of the capital naked and confess his sins to the populace.
Somewhere in the bowels of the Great Sept Lancel is being fully initiated into the Faith Militant by having the seven-pointed star carved into his forehead. He then accompanies his brothers when they go to the training grounds to seize Ser Loras Tyrell. When the knight of the flowers protests, Lancel declares he has broken the laws of gods and men, and that justice (Lancel himself) has come for him at last.
Lancel is patrolling the streets of the capital with his Militant brothers, when Petyr Baelish and his Vale escort walk up. Lancel tries to stop them, but Baelish sneers that he has urgent business with Cersei and can't afford to be detained. Lancel angrily warns Littlefinger that in the new Kings Landing 'flesh peddlers' are dealt with harshly. Unimpressed Petyr merely quips that while they both peddle fantasies, at least his bring merriment. The jape leaves Brother Lancel seething but he allows Baelish to depart all the same
Lancel, possibly with some prodding from Littlefinger, confronts Cersei during a conversation with the High Sparrow. It is soon made clear that he confessed all his sins, and the High Sparrow has her arrested at last.
For the first time in weeks Lancel is seen at Baelor's Great Sept. Fervently devoted to the Faith, Lancel and his Militant brothers silently stand beside their leader. When Jaime Lannister comes to claim Queen Margaery and Loras, things get tense, with Lancel eyeing his cousin angrily. The High Sparrow and King Tommen diffuse the situation by proudly announcing their new alliance.
Lancel and a detachment of Faith Militant brothers have come to collect Cersei for a meeting with the High Sparrow. Cersei is reluctant to say the least and becomes very flippant as per usual. When Lancel orders one of his brothers to take Cersei the Mountain rips him to shreds. This gruesome act causes the remainder of the Faith Militant to flee hastily.
When the King's Mother doesn't show up at the Great Sept, Lancel is sent to retrieve Cersei for her trial. At his way he spots one of Qyburn's Little Birds running into a dark hallway and follows the Little Bird. He then finds a huge stash of Wildfire, but is stabbed in the bottom of his spine and paralysed. He spots a puddle of Wildfire with 2 candles at the end of the tunnel. He crawls all the way to candles but fails to put them out before they ignite the Wildfire and explode, killing him and everyone inside The Great Sept of Baelor.
First cousin and a former lover of Cersei Lannister, lancel served as Robert Baratheon's squire. He swore himself to the Faith of Seven after suffering a grave injury during the Battle of the Blackwater. After Littlefinger told Olenna of Lancel's past activities, she used the information to force Lancel to confess his sins to the High Sparrow, who then had Cersei imprisoned. Lancel died uncovering the stash of wildfire Cersei used to destroy the Sept of Baelor.