Tyrion Lannister is a brilliant man who is rarely seen for anything other than his dwarfism. He is clever, quick and well learned. Many have wrongly assumed he is cruel because of his physical deformities making him appear 'cruel', but this could not be farther from the truth. Son of Tywin Lannister, brother to Cersei and Jaime Lannister.
Tyrion's mother, Joanna Lannister died birthing him, and as a result his father Tywin Lannister always resented him. His older siblings Jaime Lannister and Cersei Lannister are twins and were both eight at the time. Cersei also resented him, but Jaime always treated him well. Still Cersei and Jaime were closest to each other (later to the point of incest).
As a teenager, Tyrion and his brother Jaime found a girl being attacked on the road. While Jaime ran off the attackers, Tyrion took the girl to an inn and comforted her. They had sex and Tyrion secretly marries the girl. He is happy with her for a fortnight. It was not known by Jaime that subsequently the two were married. When Tywin found out, he was furious. He had Jaime confess that the girl was a prostitute and that he had staged the attack to make Tyrion happy. She was to pretend to act as if she was about to be raped and then to allow Tyrion to seduce her. Tywin had 50 of his soldiers have sex with Tysha in front of Tyrion, making him go last. Tyrion has never stopped thinking about Tysha, as this was the only time of his life that he was happy.
When Tyrion later finds out, in A Storm of Swords, that she wasn't a prostitute and she did genuinely love him, but that Tywin made Jaime lie about this, as he saw the girl as a gold digger, it permanently ruins the brothers' relationship.
Tyrion Lannister goes to Winterfell with King Robert Baratheon's party. He meets Jon Snow, a bastard and fellow outcast in society. Brandon Stark falls from a tower and then an assassin tries to kill him as he lay unconscious. Littlefinger identifies the knife used in the murder attempt as one that he lost to Tyrion Lannister on a bet. Bran's mother Catelyn Stark believes this and takes Tyrion prisoner at the Inn at the Crossroads. Although he was innocent, Catelyn took him to the Eyrie, where he was imprisoned. Tyrion insists on a trial by combat. There, Bronn, a sellsword, agrees to fight for his life and duels Ser Vardis Egen. After winning the duel, Bronn escorts Tyrion back to the army of his father Tywin Lannister in the Riverlands. Tyrion fights bravely in the Battle of the Green Fork and his father decides to send him to King's Landing, to act as Hand of the King. While fighting in his father's army Tyrion met Shae, a prostitute from Essos, who he falls in love with, and also accompanied him to King's Landing. He also gets a squire by the name of Podrick Payne.
Tyrion becomes the Hand of the King in the absence of his father, which angers his sister. In order to hide Shae, knowing Cersei will try to harm her in order to harm Tyrion, he sets her up in a manse outside of the Red Keep. Tyrion takes elaborate precautions to prevent being followed when he visits her. He visits a brothel and takes a whore named Alayaya up to a room. Going into a tunnel which takes him to a stable, he can then ride in disguise to Shae's manse. Later when the city is too dangerous, he has her become the maid of Lollys Stokeworth. He winds up having a power struggle with Cersei, who is acting as queen regent while her son Joffrey Baratheon is an underage king. Tyrion makes decisions to anger Cersei, including marrying her daughter Myrcella Baratheon to a prince of Dorne and trying to get Joffrey to fight in the Battle of the Blackwater. His cunning won the Battle of the Blackwater, because he consulted alchemists to use Wildfire, which defeated Stannis Baratheon and his fleet. He winds up injured during the battle and wakes up with his nose mostly gone and a hideous scar across his face. He is then replaced by Tywin Lannister as the Hand of the King.
Tyrion is appointed as Master of Coin, replacing Petyr Baelish. He is forced into a political marriage with Sansa Stark, who is repelled by him. His father wants him to consummate the marriage, but he refuses because he knows Sansa doesn't want to. Shae doesn't seem to care about his marriage, which makes him wonder if she ever loved him. Later, at Joffrey's wedding feast, they have an argument. When Joffrey is poisoned later that night, people assume Tyrion is guilty, when in fact it was Olenna Tyrell who poisoned him. After a trial by combat, he is sentenced to death, but Varys and his brother Jaime Lannister helps him escape. Jaime tells him the truth about Tysha, which makes him vow to one day kill Jaime. Tyrion tells Jaime that Cersei is being unfaithful and confesses to killing Joffrey. He does this to hurt Jaime, and it is successful. Jaime leaves him with Varys and is stunned. As Tyrion is being lead out of the Red Keep, he realizes that he is near the Tower of the Hand, and he takes the opportunity to kill his father Tywin. He flees across the Narrow Sea to Essos, and Cersei offers a lordship to anyone who can return him.
Tyrion is still crossing the Narrow Sea and regarded as a kingslayer and kinslayer, although only the latter is true. Meanwhile, in King's Landing, Cersei offers rewards for his head, but the heads brought to her are not his (therefore, the reward winds up killing innocent children and dwarves).
"Perhaps that is the secret. It is not what we do, so much as why we do it." - About Chataya.