In the Song of Ice and Fire novels, Simon was a descendant of House Toyne. House Toyne was formerly a house of honor, with one member serving in the Kingsguard of King Aegon IV Targaryen. When Aegon IV found this knight abed with one of his mistresses, he had him executed in a horrible fashion. The knight's brothers declared war against House Targaryen, slaying the infamous Dragonknight, Ser Aemon Targaryen. Simon was their descendant and the last of his line.
A member of the disgraced House Toyne, he led the brotherhood to infamy, kidnapping noblemen and earning the trust of the local smallfolk. Toyne even once competed in a tourney at Storm's End against Rhaegar Targaryen, disguised as a mystery knight.