This question contains spoilers for the second season of the Monogatari series.
Oshino Ougi refers to herself as the niece of Oshino Meme. But Kaiki mentions in Koimonogatari/Hitagi End that Meme has no family at all and it's not possible that he has a niece.
Also, this news-article at ANN suggests that she (or he?) fights with Koyomi in the latest novels and it seems to me that she is responsible for Nadeko becoming a god. Is she actually an antagonist?