Very short version of a long story below:

Long, very long ago there was a group of small sized mermaids (30cm mermaids also called Merlessnes).

Because of their size and their not very bright personality they where banished to a dark area deep in the oceans. 

The more aggresive breeds have some advantages regarding other breeds. The foodchain of the mermaids looked as followed:

-Undine

-Sirena

-Merserpent

-Mermaid

-Human born Mermaid

-Naiad

-Merlessnes (or Meerminderheden)

-(Cursed Skeleton Breed) Meerminderheid

The Merlessnes breed could reach ages up to 1200 years and because of that they have been hunted for their skin and organs. 

The skin and organs provide the holder eternal life. The backside is that the donor must be separated from 80% of the skin and all organs underneath the body.

The second requirement is that the donor should be alive, which can be aquired by magic. 

The Undine and Sirena breeds are capable to use their magic voice for that purpose. But only the Sirena does this because a Sirena reaches ages of 50 years and the very rare Undine breed is immortal. 

Serphien was a Merlessnes turned into a (Cursed Skeleton Breed) Meerminderheid by a Sirena named Hibuki. Till this day she is searching for Queen Undine Krakine who can turn her back into a merlessnes (or something better as she hopes). Or to find a solution to kill the immortal Sirena Hibuki.