Kim was a hunter , mostly kills and guards to get money as a payment. She's been like that ever since she was sold when she got kidnapped when she's still 6 years old. Been through a lot of hardship she became  like that.
She never went to school but knows a lot of stuff because of her works requirement. Kim always work from outside the country as she's the number 1 hunter in the company. But been called out to their country for a mission even after she's done with her current one.

Pie was a rich girl and a spoiled one at that. All her bodyguards and maids can't even keep up with her pace as she always trick them that's when she was a kid of course. But because of some incidents made from one of her bodyguard they're afraid to hire one again.
But her father was afraid that she might get in trouble if there's no one to protect her so he called the underworld company to hire her a body guard but all of them are already old and won't fit his criteria. One of the assistant then suggested ,that why not hire one of the hunters ? there's a few one that is young but excellent.

That was then their fate connected as Kim was the one who got hired to protect Pie.