Leafa

Leafa aka Suguha Kirigaya (voiced by Cassandra Lee Moris) is Kirito's sister, and one of the main protagonists in Sword Art Online.

Pre Adventure Series
She witnessed her brother become trapped in SAO for two years, and help him to free 300 players that were trapped in ALO. Leafa also happens to be a pro at Kendo. She even helped to comfort him when he was suffering a loss of hope in rescuing Asuna.

Adventure Series
When the Heartless attacked, Kirito lost his parents from the attack. He was taken in by his Aunt, and was raised with Leafa to be siblings. However, when Kirito found out he wasn't her brother, and left, Leafa went to find him and bring him home. Eventually she started hearing rumors about a dual wielding black swordsman traveling together with a swordswoman called The Lightning Flash at a nearby village. Hoping to find her brother, Leafa leapt out to towards the direction of where this Lightning Flash lives, believing her brother, Kazuto was in trouble by The Black Swordsman, and The Lightning Flash. That all changed when she who the Black Swordsman was learning his name was Kirito. Leafa believed if she defeated them both, they would take her to her brother, only to learn when she found the two, that Kirito was none other, than her brother, Kazuto. Leafa just stood there filled with joy at finding her brother, and taking bringing him home, but all shattered when he told her he already knew the truth. Leafa just stood their heartbroken, and crying tears sadness. Leafa, then challenged her brother to a sword fight, but lost. However, the two managed to forgive eachother and reunite as a family, and met Asuna, Kirito's wife. along with their daughter, and her niece, Yui. The family went back to Kirito, and Leafa's home village and reunited as a family.

Trivia

 * Leafa will appear in the near future.