She is a Felisiren, an invasive marine species with an unnaturally fast metabolism. They don't hunt out of cruelty. They hunt because they must.

As children, they survive almost entirely on fish, and wherever they settle, marine ecosystems collapse.

To humans, they became a biological threat.

So the government decided to erase them.

Original art is by : @kellydacheetah

Her cute/disguise/normal form

Her dark/monsterous form: