So let's start with episode 1,
in the first scene the male protagonist Yuuto is sleeping and a pink haired girl comes into his room. She says she's a vampire but looks more like a Succubus with the bat wings, the arrow demon tail and the ability to grant wishes. She says that she's gonna suck his soul...so yeah she's a Succubus.
But the series is actually getting ahead of itself so lets see how it all started:
so the boy was hired to be the servant at this mansion where lives an oldish man and 3 hot women (a blond, a green haired and a pink haired one) with huge breast and who dont wear bras.
The boy is replacing his father who the old man claims to be a criminal (maybe that will be explained in ep 2)
Now lets go back to the boy and the Pink Haired. He's supposed to die by tomorrow since his soul was sucked (actually his dick was sucked, his soul was sucked through his dick, seems like a good way to die)
Tomorrow comes and he's still alive! Could he be "The One"?!
We'll find out next episode.
So yeah he's the chosen one, not much to tell. He does end up evolving from Yuuto to Super Yuuto though, he ends up looking like Inuyasha and Demon Kurama (from YuYuHakusho).