|“||Listen up, Yūma. Whenever you fight an opponent stronger than yourself, don't try to win.||”|
–Yūgo Kuga 
He sacrificed himself to save his son, Yūma when he was about to die after being attacked and defeated by an unknown enemy. This was done by converting his life energy into a Black Trigger which sealed Yūma's dying body and replaced it.
Yūgo was a fighter of the highest caliber, and was the person who taught Yūma how to fight. His presence during the war against Spinthir was said to be extremely important for Calvaria. Before his death, he was about as strong as Shinoda during the Large-Scale Invasion Arc and Mogami's rival. He also had enough trion to be able to create a Black Trigger and possess a Side Effect. The latter allowed him to see through lies, and was passed down to his son through his Black Trigger.