Joanna Lannister

Joanna Lannister is an unseen character mentioned in the second and fifth seasons. She is deceased when the events of the series begin and is not expected to appear in the series. Lady Joanna was the wife of Tywin Lannister and the mother of Jaime, Cersei, and Tyrion Lannister. As Tywin's first cousin, she was a Lannister before her marriage.

Background
Joanna was the wife of Tywin Lannister, the head of House Lannister and the Lord of Casterly Rock. Joanna was perhaps the only person Tywin loved unconditionally. Joanna bore him the twins Jaime and Cersei. She died giving birth to Tyrion. Joanna died when Cersei and Jaime were only four years old. Tywin took Joanna's death very hard, as did Cersei, who holds Tyrion responsible for her death. However, Jaime understands that Tyrion was just an infant and had no control over what happened.

Season 2
When Tyrion arrives in King's Landing to serve as acting Hand of the King, Cersei is annoyed at how he is checking her power, and his quips that she is not ruling wisely. At one point she bitterly says that he is funny, but nothing will ever match his first joke: killing her mother in childbirth when he came out of her. The greatest joke of all is that she had to die so something as worthless as him could live. Tyrion is hurt and coldly reminds her that she was his mother too.

Cersei explains to Sansa Stark that when she was four and her mother died, she prayed for days hoping the gods would give her mother back until her father explained that prayer could not do that.

Season 3
After the Battle of the Blackwater, Tyrion (who was seriously injured in the battle but recovered) came to his father in the Tower of the Hand. He asked that as a basic reward for loyal service, his father grant him rule of Casterly Rock itself, given that Tywin would be remaining in King's Landing as Hand of the King. Tyrion pointed out that by all laws it was his right, given that his older brother Jaime foreswore all right to inheritance when he joined the Kingsguard. Tywin refuses and when Tyrion asks why, he launches into a bitter tirade saying that he shouldn't need to explain something so obvious: Tyrion's mother died so he could enter the world, and on top of that he is a stunted dwarf whose very existence is a shame to their family. Tywin always hated Tyrion for "killing" his beloved wife.

After the Red Wedding, Tyrion confronts Tywin after a Small Council meeting about his plans to force Tyrion to impregnate Sansa Stark, which he says is for the good of the Lannister family. Tyrion counters by demanding to know when Tywin ever did something "for the family", putting the family's interests above and at the expense of his own. Wounded and choking back anger, Tywin bluntly declares that the day Tyrion was born and his mother died in childbirth, he was so stricken with grief that he wanted to carry Tyrion into the sea and let him drown but stayed his hand because Tyrion is a Lannister and Tywin refused to become a kinslayer. Therefore, he earnestly believes that Tyrion should be outright grateful for this as if it were a deep personal sacrifice: that Tywin refrained from killing his own infant son.

Season 5
When Daenerys asks Tyrion why she shouldn't kill him for what his family did to hers, he answers: "You want revenge against the Lannisters? I killed my mother, Joanna Lannister, on the day I was born. I killed my father, Tywin Lannister, with a bolt to the heart. I am the greatest Lannister-killer of all time."

Quotes
"You want revenge against the Lannisters? I killed my mother, Joanna Lannister, on the day I was born. I killed my father, Tywin Lannister, with a bolt to the heart. I am the greatest Lannister killer of our time."

- Tyrion Lannister to Daenerys Targaryen

In the books
In the A Song of Ice and Fire novels, Joanna Lannister was a daughter of Tytos Lannister's younger brother, Jason Lannister, making her a first cousin of Tywin Lannister. Her surname was already "Lannister" before she married Tywin. First cousin marriage is not uncommon among the nobility in Westeros, and apparently isn't considered incest in their culture. She was the sister of Ser Stafford Lannister, who was later killed during the War of the Five Kings by Robb Stark's army.

Though Tywin was always a stern and ruthless man, Joanna was said to bring out what little warmth he had, and he was even known to smile in her presence. Tywin and Joanna were very deeply in love, and thus Tywin took her death very hard. He became even more stern and was never known to smile again. Her death also fueled his unreasoning hatred for his younger son Tyrion, whom he blamed for killing her, combined with the shame he brought on their House for being born a dwarf.

Tywin later wanted to force all of his children into marriage-alliances to benefit House Lannister (as Cersei was later married off to King Robert), and he was offended that Tyrion risked such a future marriage alliance for himself by marrying a commoner for love when he was young (Tysha). The hypocrisy in this is that Tywin himself married for love when he wed his own first cousin Joanna, instead of entering into a marriage-alliance with another powerful noble family. As a minor Lannister cousin, Joanna brought no new alliances, lands, or wealth with her when she married Tywin.

According to Barristan Selmy, the Mad King lusted after Joanna. At the wedding of Tywin and Joanna, Aerys drunkenly japed about how it was a pity the First Night tradition was banned and took too many liberties during Joanna's bedding. Tywin was not amused, and that caused further friction between him and Aerys.

According to the books, Cersei and Jaime were eight or nine years old when their mother died, while in the TV series Cersei said she was four years old.