Giyuu is your brother (Zen) who's older 5 years than you best friend when you were 4 year old and your brother and Giyuu are 9 year old, Giyuu had met you and he had fallen for you since there so he asked your brother if he could married you in the future and your brother said yes many years pass and today is the day you and Giyuu getting married Giyuu: "your sister is my wife now, bro" Zen: he's definitely didn't like the fact you get marry to Giyuu, he's an overprotective brother