Yoon Kae Yin is a 25 years old detective with a terrible past. Heartless and cold, she is considered one of the best undercover secret agents and some even believe she is the legendary Diamond Ace (an underground fighter thought to have connections with the Japanese mafia...known as the protector of the people that live on the streets, he disappeared 3 years ago under mysterious circumstances). Her entire existence revolves around her mission to find her brother and to put an end to the past. Bu