Some snip set of the character in this FF. 
To get the clear picture of the character based on current story (I will update it again later)
 

Kim Jong Kook (30) --- Father + Mother (died 25 years before) – work as police – also known as Detective Superintendent Kim Jong Kook (D.S Kim). 

Song Ji Hyo (27)  --- Father + Brother (Joong Ki) – works as police (Inspector) – disguise as Cheon Sung Im -- have mission given by her father

Song Jung Woo ---- Daughter (Song Ji Hyo) + Son (Joong Ki) + 3 best friend (Jeong Hun, Sung Hoon & Yeong Jin) --- accident (paralyzed)  -- singlefather (wife died 12 years before)

Song Joong Ki –-- Father + Sister (Song Ji Hyo) + best friend (Lee Kwang Soo)

Lee Kwang Soo – Father + Step mother + best friend (Joong Ki)

Lee Young Jin (died) --- best friend of Song Jung Woo --- death still in mystery --- wife (Choy Nam Seon)  + missing son 

Choy Nam Seon --- husband (died) + missing son --- accident (lost memory) --- married again --- step son (Lee Kwang Soo) – known as Ahjumma by Ji Hyo & Joong Ki --- acquaintance of Song Jung Woo

Cho Jeong Hun (Commisioner General)  - Best friend of Jung Woo, Young Jae, Sung Hoon – 19 years before had ask help from Jung Woo & Young Jae to become spy inside gangster clan. 

Kim Sung Hoon – Chief of Ilsan Police Station (ex-chief of Song Ji Hyo) – Best friend of Jeong Hun, Jung Woo and Young Jae

Kang Gary – police at Ilsan Police Station – best friend of Song Ji Hyo – had crush on Song Ji Hyo

Kaonashi --- serial killer who seek for revenge 

Kaonashi victim

Yeol  Jee Gun – leader of Yeol Clan – Inherit from his father (died 19 years before)

Kim Jae Hee – Yeol Jee Gun’s driver

p/s : Who is Kaonashi????