Oberyn Martell

"Tell your father I'm here - and tell him the Lannisters aren't the only ones who pay their debts."

- Prince Oberyn Martell to Tyrion Lannister

Oberyn Martell is a character that debuted in the fourth season and is portrayed by Pedro Pascal. He's a member of House Martell, the ruling house of Dorne.

Prince Oberyn, known as the Red Viper of Dorne, is the younger brother of Doran Martell, the Prince of Dorne, and the late Elia Martell, wife of Rhaegar Targaryen, the late Prince of Dragonstone and heir to the Iron Throne.

Background
Prince Oberyn is the younger brother of Prince Doran Martell and Princess Elia Martell; like most members of House Martell, he is styled "Prince" instead of "Lord". Oberyn is known for being an accomplished warrior, but Tyrion implies that he is just as infamous for his sexual appetite. Oberyn's official paramour is Ellaria Sand.

Season 4
Tyrion Lannister meets the lords of Dorne outside King's Landing, only to find out that Oberyn will be representing House Martell at the king's wedding, and not his brother, reigning Prince Doran. The Dornish lords tell Tyrion that Oberyn in fact arrived in the city before dawn. It doesn't take Tyrion long to realize that Oberyn is in one of Littlefinger's brothels. Oberyn assists his paramour Ellaria in choosing a whore for her own enjoyment before selecting Olyver to satisfy himself. Oberyn's fun is cut short when he hears Lanister men singing the Rains of Castamere, which sends Oberyn into a quiet rage. Before one of them could draw his sword, Oberyn drives his dagger through the man's wrist, though he assures the man's compatriot that he can be saved, if he receives immediate medical attention. Tyrion arrives, and interrupts the fight. Leaving the brothel, Tyrion asks Oberyn why he came to the capital. Oberyn, much to Tyrion's discomfort, relates the story of how Gregor Clegane, Tywin's bannerman, raped and murdered Elia Martell and slaughtered her children. He then warns Tyrion that the Lannisters weren't the only ones who paid their debts.

Quotes
"If the Mountain killed my sister, then your father gave the order."

- Oberyn to Tyrion Lannister

"You're not a lion. You're just a pink little man who's far too slow on the draw."

- Oberyn to a Lannister soldier

"The Lannisters aren't the only ones who pay their debts."

- Oberyn to Tyrion Lannister

In the books
In the A Song of Ice and Fire novels, Prince Oberyn is the hot-headed younger brother of Prince Doran Martell. He has eight bastard daughters, called the Sand Snakes, the four youngest of which by his current paramour, Ellaria Sand.

Oberyn has a lined face with thin eyebrows, black "viper" eyes and a sharp nose. His hair is lustrous and black with only a few silver streaks and recedes from his brow into a widow's peak. It is rumored that he has sexual relations with his squires when Ellaria is unavailable. It is not unusual for lords and ladies in Dorne to be openly bisexual, due to the relaxed attitude towards sexuality among the Dornish.

Oberyn is called "the Red Viper" by friend and foe alike due to his penchant for coating his weapons in lethal poisons, so that even a glancing wound will prove fatal.