Dorna Lannister

"Your mother was a dumb whore with a fat arse, did you know that?"

- Robert Baratheon to Lancel Lannister

Dorna Lannister née Swyft is an unseen character in Game of Thrones. She is the wife of Ser Kevan Lannister and mother of Ser Lancel Lannister.

Background
Dorna is a daughter of House Swyft of Cornfield. She married Ser Kevan Lannister, younger brother of Lord Tywin Lannister. Her husband's niece, Cersei Lannister, insisted that her husband, King Robert Baratheon, make Dorna and Kevan's son, Lancel, his squire.

Season 1
King Robert informs Lancel that his mother was a whore with a fat arse, to Lancel's discomfort and distress. Robert was angry with Lancel and presumably not offering a reliable appraisal of Dorna Lannister's character and appearance.

In the books
In the A Song of Ice and Fire novels, Dorna is the daughter of Ser Harys Swyft, the Knight of Cornfield and a loyal bannerman of House Lannister. Her father married her to Kevan Lannister and they have three sons: Lancel and the twins Martyn and Willem. They also have a daughter, Janei. Lancel is their eldest child and Janei the youngest.

According to Kevan, Dorna is a gentle soul, never comfortable except at home with friends and kin around her. She prays seven times a day and loves needlework and flowers.

Dorna is described "chinless lady, flat-chested and chicken-legged" with her lack of chin shared by her father, yet she is still very loved by her husband.

It is unclear from Robert's comment about Dorna "dumb whore with a fat arse" whether her physical description was changed in the series (as done with many characters) or he called her that way just to insult Lancel.