She didn't know she was half-blooded until she reached the age of 16. Somi was the daughter of a human and a pure white caster; thus making her actually a really powerful caster. She grew up, had a normal life until things started to get weird when she turned 16.

It started that day, when he was assigned to protect Somi from the monsters, the black casters who wanted to kill her because she was half-blooded. He had to train her and to make sure she was ready to be a fully powerful caster. 

When things were going upside down, what happened when love took place?