Lord Petyr "Littlefinger" Baelish is a major character of A Song of Ice and Fire.
He grew up poor in Riverrun with Catelyn Stark and Lysa Arryn, and fell in love with Catelyn there, while Lysa fell in love with him. When Catelyn was engaged to Brandon Stark, Petyr challenged him to a duel for her hand. Catelyn gave Brandon her favor and shunned Petyr, and he lost the duel. Petyr left Riverrun in shame and later became Master of Coin for Robert Baratheon and the Iron Throne.
He is well-known for playing false and having his own best interests in mind; when Eddard Stark came to King's Landing to be Robert's Hand, he sought help from Littlefinger. After sneaking Catelyn into King's Landing and then smuggling her back out, Petyr turned around and betrayed Ned to Cersei Lannister (though he did warn Ned not to trust him.) Petyr later reveals to Tyrion Lannister that he took Catelyn's and Lysa's maidenhood, and in A Storm of Swords, he goes to the Vale to convince Lysa Arryn to join the side of the Iron Throne in the War of the Five Kings. Instead of winning Lysa to his cause, he stays to court her (as she has never fallen out of love with him). Later, he spirits Sansa Stark away from King's Landing and the Lannisters to bring her to the Vale. Petyr claims that Sansa is his bastard daughter "Alayne" to protect her from those who would harm her. He has always loved Catelyn but she is dead now and he starts falling in love with her beautiful daughter Sansa who is "more beautiful than she ever was" says Petyr comparing her with her mother.and later kisses her. When Lysa finds out, she threatens to kill Sansa, but Littlefinger has her killed It is in this scene that Sansa learns that Littlefinger convinced Lysa to kill her husband Jon Arryn.