The main protagonist of the series, she is initially portrayed as a kind, clumsy and helpful person, acting very cheerful and singing songs in the morning. It is revealed at the end of the series that her true self is much more serious and not at all like how she was shown at first. Fumito Nanahara states that she does not care for humans, and wishes to be able to drink their blood, but is unable to due to a contract made with an unknown person.
No one knows if that is true. But people who know about the "Old One's" tend to see Saya more as one of them as a human. Because she can unleash a power who exceeds the limits of her human shaped body.
If Saya is an hybrid-child between a human and an Old One is uncertain, or if Saya is simply an Old One with a perfect human body shape. Or a Vampire even.
But the complete conscious and serious Saya doesnt seem interested in drinking Blood at all.
What is certain is that some people brainwashed her in the past and tried to use her DNA to become something different as well, something that isnt human anymore. Those people played with her feelings and the person who seems to be in charge of them all is known as Nanahara Fumito.
After the destruction of the Town they used to play with Saya's feelings and mind, Nanahara Fumito revealed everything to her, making her chase after him in what seems an attempt to avenge her dead friends.