ahhh, the 'we will be together forever'-promise :/ even as a pinky promise :/
how much you wanna bet, that they not only going to be seperated in the end, but it having something to do with that mermaid magic stuff?
like our one MC will find something precious that belongs to the other girl by the beach after running after her to there for some reason and then she'll all be like:
"That day our promise was broken. Years past and I never saw her again. Maybe she found the mermaid she was looking for and followed her into the sea..." etc etc and then we will all be left wondering if there was some actual mermaid or wether or not.she drowned herself in some kind of delusion :/
and it'll be also like "everybody else got married and even had children. I stayed alone. I could feel the same about anyone else ever again. She was my one love."