Syrio Forel is a master sword-fighter and instructor hired by Eddard Stark to train Arya Stark.
Syrio Forel is a bravo and master sword-fighter, originally from the Free City of Braavos where he spent nine years as the First Sword of the city, before relocating to King's Landing.
After arriving in King's Landing, Lord Eddard Stark discovers that his daughter Arya owns a sword (Needle, a gift from her half-brother Jon Snow), but has not been properly trained to use it. He hires Syrio Forel to train Arya in its use. Forel accepts the commission and begins to train Arya in how to use the sword in the Braavosi style, known as the Water Dance. Arya's lessons are kept a secret; her sister Sansa believes that Arya is getting "dancing lessons".
Eddard finds Arya balancing on the top of some stairs. She tells him that Syrio set her the task to improve her balance. Arya chases cats to improve her dexterity and speed on Syrio's suggestion.
Syrio and Arya have another lesson, but Arya is distracted. Syrio tells her that she is more vulnerable when she is "with her trouble" rather than concentrating on the here and now. He teaches her how to forget her troubles so she can concentrate on fighting. After Eddard's arrest for treason, Lannister guards led by Ser Meryn Trant of the Kingsguard arrive to take Arya into custody. Syrio informs them that he will protect her. When they try to barge past him, he disarms one guard with a training sword. He fights off a mass attack by the guardsmen and then engages Ser Meryn, telling Arya to run. When she asks him to come with her, he tells her, "The First Sword of Braavos does not run." Arya flees, and (unseen) Syrio and Ser Meryn continue their fight.
Arya believes Syrio to be dead, and tells the Hound that he was killed by Ser Meryn Trant. Sandor proceeds to mock Syrio, claiming that Trant is a poor fighter, and that anyone could beat him as long as they have armor and a sword.
A vengeance is taken for Syrio at last, when Arya kills Meryn Trant, out of revenge for whom she believes killed Syrio after the arrest of her father.
Ned Stark hired Syrio Forel, the former first sword of Braavos, to give his daughter "dancing lessons" after she became aware that Arya had a sword (Needle) she had not been trained to use. When Lannister guards tried to seize Arya during a lesson, Syrio held them at bay so she could escape.