Nana underestimated Tsukasa far too much... Tsukasa knew that, which is why she easily manipulated her.
I feel that Tsukasa's whole life (if she ever gets tired of Nana) will just be a cycle of her exploiting other women. At least until Tsukasa meets a crazy girl who either match her freak, or someone even more mentally unstable than Nana who will eventually kill both of them in the end.