I thought it'd be appropriate that my first submission on Furaffinity is my first anthro character I ever made!

Gisslin is actually a human turned into the creature you see. The series she's from actually explains it all. I'll give you a short cut of it, though.

A planet named Aurror was full of greedy people and constantly tried to seize control of their neighboring planet, Zealgi. Aurror's scientists built a machine that emitted a sonar-like frequency that could (bear with me here) disrupt the native's DNA and transform them into feral beasts; sort of like deevolution through sound. Gisslin was affected, but like some other Zealgians, her humanity remained intact. Much later, she meets the Main Character and decides to join him in stopping Aurror's growing ambitions.

As a side note, the animals they become are called "Spirals" a portmanteau of Spire and Ferals.