in this story, supposedly, when a human drowns and a siren sees the human, they make a contract. this contract makes them switch places. the human turns into a siren and forgets his/her human past while the siren that kissed the human instead of staying a siren becomes human.
but, this time, that situation didn't happen. the siren became human but is still a siren and the main character becomes a siren but is still human with all her memories.
so, like chapter 4 showed, when they are in the ocean (or water) they will be sirens but on land, they will be humans.

Now she's a mermaid too.. Dii I miss some parts there?