Thoros of Myr is described as a tall, fat bald man who wears loose robes. He is very old by the time he joins the Brotherhood Without Banners.
Thoros of Myr was the youngest of eight children and handed over to the priests of R'hllor as a little boy. He came to King's Landing, hoping that he could convert Aerys II Targaryen to worshipping R'hllor because Aerys was so obsessed with fire. However, it only made him question his own religion, as he was never extremely pious to begin with.
He was a good friend of King Robert Baratheon and was one of his primary drinking partners. He is renowned for his fighting in the Greyjoy Rebellion, being the first through the walls of Pyke after they fell, with his iconic flaming sword ablaze.
He is a competent tourney fighter, particularly shining in the melee, where he uses a sword covered in wildfire to beat his opposition. He utilizes this weapon to defeat Sandor Clegane 3 times, because Sandor is afraid of fire. Thoros was defeated by Lord Yohn Royce with a normal mace after the fire on Thoros' sword went out.
A Game of ThronesEdit
Thoros rode in the Hand's Tourney, unhorsing Lord Beric Dondarrion. He wins the melee event afterwards and was then named in the group sent under Beric's command to bring Ser Gregor Clegane to justice. He fights in the Battle at the Mummer's Ford, helping Edric Dayne to protect Beric's corpse and giving Beric the last kiss, a tradition in Thoros' old religion, that brings Beric back to life.
A Storm of SwordsEdit
Thoros and Beric start the group known as the Brotherhood without Banners, and Thoros converts most of them to be followers of the red god. The group stands up for the smallfolk that have suffered as a result of the War of the Five Kings and fights both 'Wolves' and 'Lions' equally. Any prisoner taken is put on trial before the red god to receive judgement. Thoros has something of a spiritual reawakening as he finds that things he couldn't do before are now possible, such as repeatedly reviving Beric.
A Feast for CrowsEdit
Thoros doesn't take a leadership role after the permanent death of Beric. he allows Catelyn Stark, now known as Lady Stoneheart, to take control of the Brotherhood, even though he personally disagrees with her motives. Thoros loses confidence in the cause as a result of this.