Tis a simple problem to solve! just have your favorite anime femal stumble
onto some mermaid flesh! See simple! They could find it in a box on the ground
or have some one give it to them, either as thanks for saveing them or in
attempt to kill them (It kills most people. A select few it turns into lost
souls, and an even more select few become imortal.)  Really any magical device
will do. You can even have it break or be destroyed (or even just stolen if
you want to keep it in the story.) (if they are a powerful martial artist you
can have it knock them out when it makes them imortal and wake up with it
already gone or something.) after the character uses it.
Ja ne
Catbert25