Tristan, formerly known as Simon, is the Commander of the Warrior's Sons.
Simon grew up in a poor fishing village along the Stony Shore. Missing a mother, he was often abused by his father, being beaten often. One evening the drunken father came stumbling back to the collection of wooden planks that they called home, and when he saw his son eating a potato, one that should have been reserved for him, he drew his knife in a fit of madness and cut the boy’s face, leaving him with a terrible scar reaching from his right cheek to his left ear, cutting his nose in half.
One day, at the age of twelve, he was walking with a basket of fish to sell at the market, when his tormentors came along. As usual, they threw him on the ground an began kicking and beating him. What wasn’t usual, was that Simon had the knife he skinned the fish with in his basket. Normally, Simon would let himself get beaten, but this time he reached for his basket, drew the knife and stabbed one of the boys in the belly. He fled after this incident, wandering around Westeros. He wandered the Seven Kingdoms until he stumbled upon a band of outlaws. They adopted him, and Simon's days were filled with rape, murder and theft.
one day they were surprised in the middle of the night by a knight and a good fifty men-at-arms. His friends were slaughtered, but a few managed to escape, including Simon. Together, the few escapees started a small company of sellswords, consisting of six men. When the Second Greyjoy Rebellion started, they joined the war against the Ironborn. There he was knighted by a knight named Tristan, but knighthood didn't last long before he was knocked down the slopes of the shores and drowned. After he survived this, he claimed to have been saved by the gods, and Simon became a good man, living like a knight should.
Simon was in the Westerlands when he heard of a Septon who started the Divine Company. Still feeling guilty for his past sins, Simon felt forced to atone for them. He went in search of this Septon, and when he came upon him, he joined his holy order. Being one of the first to do so, and being a promising warrior, Simon became Commander of the Warrior’s Sons at the age of 42. As is custom in the order, Simon had to change his name. This suited him just fine, as he would abandon the name with which he raped and murdered. His new name would be Tristan, after the knight who had knighted him. His men call him Commander Tristan.
Simon joined the Divine Company, becoming leader of the Warrior's Sons, and being renamed in the light of the Gods to Tristan.