Leafa is Kirito cousin/sister the anime Sword Art Online : ALfheim . She is also a recruit able npc in Sword Art Online : Hollow Fragment for the PS Vita video game.
Warning spoiler don’t read if you haven’t watch the anime or novel.
Kirito is actually an adopted son. His parent passed away when he was a kid and his mother’s sister took him as their child. According to the novel both Kirito and Leafa knows about this matter , which is why Leafa action towards Kirito is more to ‘being in-love’ than ‘being a sister’.
Like any other male character in an anime, Kirito is dumbstruck oblivious towards Leafa affection.
The model of Leafa here is not a perfect rip.