Well, if Tsukune summoned her by accident...someone messes up his parameters of the spell he's casting or just some crazy fluke, and the spell turns into a summoning portal...lol Can even make it Harry Potter-ish in that he's in a magic school or something. Moka's huge aura distorts magical energy for a good radius around her - S-class monsters/demons aren't suppose to be summoned, as in, they technically
can't be summoned. Could use a 'destiny' thing to have the two bound together - his blood on the holy rosary basically seals her vampire aura but she still remains as Inner, she's just reduced to a mere 30% of her full power. She can't touch him because of his blood used as a catalyst on the silver sealing her. Yet, only he can release her when he wants to.

So she's an unwilling servant to him....
...

Oops, did it again.