He has been tortured by every single torture tool in the Blood Fiend Palce, which is porbably the cause of his extremely twisted and brutal personality.
He is very unsettling especially for those he deems unimportant or his enemies as he always has a cheerful smile around them.
His mother was of the Nine Temples, the Young Lady Lord of the Spirit Void Temple and was recognized as the most beautiful person throughout the entire Nine Temples and Twelve Palaces. Gu Yu was one of her pursuers but changed personality immediately upon marrying her. She ended up passing a silent death, almost unbeknownst to the world.