Lord Petyr Baelish, nicknamed "Littlefinger," serves as Master of Coin (treasurer) of the Seven Kingdoms at the beginning of the series. He is described as handsome, but quite short. Though a lord, his ancestral holding is an extremely small and poor island.

Petyr is a childhood friend of Catelyn and Lysa Tully. He was in love with Catelyn, but she rejected him, Lysa on the other hand was in love with Petyr and got pregnant, after which Lord Hoster Tully banished Petyr from his house and had Lysa drink a potion to abort the pregnancy. Petyr started working with finances and through financial brilliance and intrigue quickly rose to the position of Master of Coin. Petyr is the mastermind behind much of the political turmoil that occurs in the novels, such as the assassination of Jon Arryn. For recruiting House Tyrell to the Lannister's side, Petyr received Harrenhal, the largest castle in the realm, and nominal authority over the war-torn Riverlands, which technically made Baelish a high lord. His new status allowed him to marry the widowed Lysa Arryn, who still loved him. After marrying Lysa he murdered her and took control of the Vale of Arryn.

Petyr is also responsible for smuggling Sansa Stark out of kings landing after Joffrey Baratheons murder. He brought her to the Vale, disguised as a bastard daughter of him. He has expressed his love for her, saying she resembles her mother and stated he has plans to help Sansa regain the North.

From "A Song of Ice and Fire" by George R R Martin