Suguha, also known as Sugu, is Kirito's "sister" in the real world; she is in actuality his cousin, as her mother had been raising her sister's child from a young age, though neither Suguha nor Kirito knew about this until later [REF].
Coincidentally, she meets Kazuto's avatar, Kirito, in Alfheim Online after he saves her from an attack by player killers.