Joanna Lannister

"You've always been funny, but none of your jokes will ever match the first one, will they? You remember, back when you ripped my mother open on your way out of her, and she bled to death."

- Cersei Lannister to Tyrion Lannister.

Joanna Lannister is an unseen character mentioned in the second season. She is deceased when the events of the series begin and is not expected to appear in the series. Lady Joanna is the wife of Tywin Lannister and the mother of Jaime Lannister, Cersei Lannister and Tyrion Lannister.

Background
Joanna was the wife of Tywin Lannister, the head of House Lannister and the Lord of Casterly Rock. 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.

Season 2
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.

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. She was the sister of Ser Stafford Lannister.

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 more 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.