Tysha is a crofter's daughter, and the first wife of Tyrion Lannister.
Tysha was being attacked on the road when Tyrion and Jaime Lannister arrive and run them off. Tyrion takes her to an inn and they end up having sex. Tyrion falls completely in love with her and finds a drunken, wandering septon to marry them. He buys them a cottage, and they live together as man and wife for two weeks.
When the drunken septon sobers up, he tells Lord Tywin Lannister about the marriage. Tywin is furious that a social climber of common blood had married into the noble House Lannister. Jaime is unaware that the marriage had taken place, but his father tells him that the girl is just after money, and they need to convince Tyrion to give her up for his own good. He then convinces Jaime to lie to Tyrion, to tell him that Tysha is a prostitute that he had hired to make a man of Tyrion. Jaime thinks that the girl will only be given money and sent away after the annulment. He has no idea that, before sending her off, Tywin has fifty of his guards rape her and then forces Tyrion to have sex with her as well.
Tyrion never stops thinking about Tysha. He thinks about her whenever he is in a situation of strife. He believes that she was lying about loving him, but this was the only time in his life that he was truly happy. He has always been close and admiring of Jaime, and he loved him for going to the lengths he had to give Tyrion his first experience with a woman, even though it ended so horribly wrong.
When Jaime frees Tyrion from the cell where he is awaiting execution for Joffrey's murder, he decides to finally tell him the truth. He may never see Tyrion again, and he wants to confess to the wrong he did him, also he wants to ask if Tyrion killed Joffrey and wishes to give a truth for truth in return. He tells Tyrion that Tysha was not a prostitute and that their father simply wanted to annul the marriage without Tyrion objecting. He says that Tywin had told Jaime that Tyrion would thank him someday.
Tyrion's rage is extreme. He vows to someday kill Jaime, and delays his escape from the Red Keep in order to first confront his father. Holding a crossbow, he asks him where Tysha is now, but Tywin has no idea and doesn't take Tyrion's threat seriously. Tyrion kills him with the crossbow when he again refers to Tysha as a whore.