Not a review, so much as a bit of clarification as to what's going on here. For those of you who are confused by all this, read the manga. :P
Naw, actually, the girl Yakamo loves is Pai, who was sealed by Oshio, the snake demon. Oshio did fall in love with Yakamo though, as her memory was also damaged, in that sealing.
In the manga, Oshio becomes a kind of rival to Pai in that respect, but yeah. If you'll remember, Pai and Yamako were already in love before her consciousness was sealed by Oshio. In the american dub of this anime, it implies that Pai never existed, and that Oshio was the second personality. This is down to poor translation.
The original japanese however does keep better to the plot of the manga. Oshio does "change" but she does not become completely human. Rather, she becomes a recurrent character who can shapeshift at will.
Incidently, Yakamo becomes stronger as the books progress, even taking some of the powers Benares has. This series only covers about the first 3 or so books of the manga, and Yakamo and Pai have many further adventures before the true final.