As a boy of three, Thaddius and his brother Damon were sent to be fostered under Alannys Greyjoy in the Iron Isles by their father. On the harsh islands, Thaddius prospered. He had a morbid fascination with death and killing that lasted even after he left Pyke, and he dedicated himself to learning how to fight with a sword and a bow. He learned fast and grew faster, soon surpassing his older brother in height. At the age of ten and six Thaddius became a squire to a Knight in King's Landing. His skill was renowned throughout the realm and in a little over a year he was invited into the ranks of Knights of the Kingsguard.
Thaddius accompanied King Harys Baratheon to the tournament at Harrenhal, where he and his brother Damon met Edmure and Jojen Stark. After the melee, Jojen and Thaddius entered into an argument before deciding to get drinks at a tavern in a nearby town. After a night of heavy drinking, the two ended up sleeping together. Given the bad blood between their houses and especially their brothers, they decided to keep their romantic feelings for each other a closely guarded secret.
Thaddius marched with King Harys to Oldtown in search of the missing Maude Tyrell. After Troy Tyrell broke the guest right, King Harys ordered Thaddius to escort him back to the capital. Thaddius abducted Benjen and Mellara Tyrell as well, taking all three back to the capital as prisoners. Along the way, he forced the brothers to duel to the death, and Benjen was killed.
When he arrived at King's Landing, the city had just been sacked by the Lannisters. With his brother as King and his father Loren Lannister as the Hand, Thaddius was named Lord Commander of the Kingsguard.
Thaddius was distraught to learn that his brother had married Aeslyn Targaryen, who had given him her favor at Harrenhal's tournament. Confused about his feelings regarding the newly crowned queen and his Stark lover, and now ignored by his once doting father in favor of his kingly brother, Thaddius reached a breaking point.
After Damon scolded his brother for his cruel treatment of the Tyrell prisoners, Thaddius sank deeper into unhappiness. When the North came to the capital to fight for Damon's claim, he reignited his romance with Jojen Stark, who was now the Lord of Winterfell.
After the Battle at the Kingswood, the King went south to treat with Dorne. In his absence, Thaddius assisted in the arrest of the adulterous queen before taking advantage of the commotion of her trial to fake his death and flee to Winterfell with Jojen Stark, where he posed as a new Master of Arms for the castle.
In a later attack on the fortress during Jojen's absence, Thaddius slept with a woman, believing himself about to die. This one night affair led to later birth of Tygett Hill, Thaddius' bastard son.
After a fight with Jojen, Thaddius returned to King's Landing. He told Loren Lannister that the rumors about him were false, and that Damon had ordered him away out of jealousy. Loren died of a heart attack while confronting Damon about the matter. A bribe to the High Septon released Thaddius from the vows of the Kingsguard, and after Ulrich Dayne was tried for treason Damon sent his brother to Starfall in search of Dayne's fled squire, believed to be in possession of the King's secrets. Thaddius did not find Edmure Mallister, and after an altercation with Cailin Dayne returned to the capital in time for the Tournament of the Hand.
There he was reunited with Jojen Stark, who had entered the tournament as a mystery knight. Jojen convinced Thaddius to return North with him just before Gylen Hightower's declaration of independence from the throne.
After learning of the outbreak of war in Westeros between the throne and the Reach, a guilt ridden Thaddius sought to aid his brother by forcing Jojen to call the North's banners. Jojen refused, and the two entered into a heated argument that turned violent, resulting in the apparent death of Thaddius.
In the fifth era, it was revealed that Thaddius survived his wounds and recovered slowly at Winterfell, where he clashed frequently with Symeon Stark and Ysela Stark, then later Lyanna Stark after her visit home. When Gareth Umber and Olyvar Bolton were returned their rightful seats at Jojen Stark's command, they traveled to Winterfell for a feast. Thaddius continued to cause trouble, fighting with the Umbers in particular. He assaulted Talisa Umber.
Later, at the feast, Thaddius was poisoned, with no clear culprit.
"You know, I find I enjoy most sports. And the ones that involve people dying really are my favorite." - Thaddius Lannister
"Spear, sword, axe, bow, you name it, Thaddius can kill with it and make it look as simple as breathing." - Damon Lannister
"Thaddius looked bored half to death standing there in that white cloak. It looked to be choking the life out of him." - Blood and Whispers (Chapter 1)
"Thaddius killed his first man at eight." - Damon Lannister
"The youngest Lannister son had always been a willful boy, and never happier than when he had a sword in his hands. A life of servitude seemed ill-fitted to the man, yet on his seventeenth nameday he had donned the white cloak and taken the vows." - Blood and Whispers (Chapter 1)
"I've never been able to control him. Gods know I've tried. Knighthood, the Kingsguard, a lordship. No cloak or holdfast will contain him. Thaddius does as he pleases, and what pleases him changes at a whim." - Damon Lannister
“If you are looking for the kraken blood in your new kin, look no further than Thaddius. The boy has always had a cruel streak. His mother showed no interest in him, only Damon." - Jeyne Lannister
"The Warrior was Thaddius, all sword and muscle and reckless courage... but hadn't [Thaddius] proved more of a monster than any other man Damon had known?" - thoughts of Damon Lannister
"One time a traveling troupe visited the Rock with real lions. They had three of them, and a man would stick his head in the biggest one’s maw for their final act. Ashara always liked entertainers, but Thaddius was disappointed with any spectacle that didn’t end in bloodshed.” - Damon Lannister
Loren Lannister, father (deceased)
Gwynesse Lannister, mother (deceased)
Damon Lannister, half brother
Ashara Lannister, sister
Tygett Hill, son