Question: Hafþór Júlíus "Thor" Björnsson, is an Icelandic professional strongman, actor, and former professional basketball player, he plays which fictional character in the "A Song of Ice and Fire" series of fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones"?

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Jon Snow (character): Jon Snow is a fictional character in the "A Song of Ice and Fire" series of fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones".  He is a prominent point of view character in the novels, and has been called one of the author's "finest creations" and most popular characters by "The New York Times".  Jon is a main character in the TV series, and his storyline in the 2015 season 5 finale generated a strong reaction among viewers.  Speculation about the character's parentage has also been a popular topic of discussion among fans of both the books and the TV series.

Gendry: Gendry is a fictional character in the "A Song of Ice and Fire" series of fantasy novels by American author George R. R. Martin, and its television adaptation. " Game of Thrones".  He is a non-POV character, first appearing in 1996's "A Game of Thrones", and subsequently appeared in "A Clash of Kings" (1998), "A Storm of Swords" (2000) and "A Feast for Crows" (2005).  Gendry is an apprentice blacksmith in King's Landing, and an unacknowledged bastard of King Robert Baratheon.

Cersei Lannister: Cersei Lannister is a fictional character in the "A Song of Ice and Fire" series of fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones", where she is portrayed by Lena Headey.  In the novels, she is a point of view character.

Tyrion Lannister: Tyrion Lannister (also referred to as "the Imp" or "the Halfman") is a fictional character in "A Song of Ice and Fire", a series of fantasy novels by American author George R. R. Martin and its television adaptation "Game of Thrones".  Based on an idea that came to Martin while writing the 1981 novel "Windhaven", Tyrion has been called one of the author's finest creations and most popular characters by "The New York Times".  Martin has named the character as his favorite in the series.

Gregor Clegane: Gregor Clegane, nicknamed "The Mountain That Rides" or simply "The Mountain", is a fictional character in the "A Song of Ice and Fire" series of fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones".  In the books, the character is initially introduced in 1996's "A Game of Thrones".  He subsequently appeared in "A Clash of Kings" (1998), "A Storm of Swords" (2000) and in "A Dance with Dragons" (2011).

Catelyn Stark: Catelyn Stark (née Tully) is a fictional character in the "A Song of Ice and Fire" series of fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones".  She is a prominent point of view character in the novels.

Olenna Tyrell: Olenna Tyrell (née Redwyne), also known as "The Queen of Thorns", is a fictional character in the "A Song of Ice and Fire" series of high fantasy novels by American author George R. R. Martin, portrayed by Diana Rigg in its television adaptation, "Game of Thrones".  Olenna is first mentioned in "A Game of Thrones" (1996) and first appears in "A Storm of Swords" (2000).

Jaime Lannister: Jaime Lannister is a fictional character in the "A Song of Ice and Fire" series of fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones".  He becomes a prominent point of view character in the novels beginning in "A Storm of Swords" (2000).

Margaery Tyrell: Margaery Tyrell is a fictional character in the "A Song of Ice and Fire" series of high fantasy novels by American author George R. R. Martin, and its television adaptation "Game of Thrones".  Though a supporting character in the books, she played a more prominent role in the HBO series, and rose to become a main character eventually, where she is portrayed by Natalie Dormer.  Margaery is first mentioned in "A Game of Thrones" (1996) and first appears in "A Clash of Kings" (1998).

Hafþór Júlíus Björnsson: Hafþór Júlíus "Thor" Björnsson (] ; also Hafthor Julius Bjornsson, born November 26, 1988) is an Icelandic professional strongman, actor, and former professional basketball player.  He plays Ser Gregor "The Mountain" Clegane in the HBO series "Game of Thrones".

Supporting paragraphs: Hafþór Júlíus Björnsson Gregor Clegane
Answer: Gregor Clegane