Time Between Dog and Wolf

Soo Hyun loses his mother to Mao, the killer of the Thailand gang Green Bang, as a young boy. Afterward, a NIS agent adopts him as his son, and Soo Hyun grows up to be a NIS agent himself. During an op, he figures out who Mao is and goes undercover as a member of the Green Bang. Faking his own death, Soo Hyun moves to Thailand and uses his undercover identity as Kay, but during that time, Ji Woo gets engaged to Min Ki, Soo Hyun's adopted brother and best friend. What ending will Soo Hyun's fate, which started with a tragedy and led to a path that made him throw away his own name, see?