Jack is the son of the Japanese Emperor who ruled the area where Aku originally appeared on Earth, and is banished to the future by Aku during their first battle, where he is left in every episode to search for a way home. He was born on the day that his father defeated Aku and he seems to be the only mortal (aside from his father) to be a match to Aku. As a boy, after his father was captured by Aku, Jack traveled around the world to prepare both physically and mentally for his confrontation with Aku. He studied under various scholars, such as Egyptian thinkers, and mastered each art of combat from the cultures he met, training with African Zulu warriors, Viking sailors, Robin Hood, Mongolian warriors, Shaolin monks, Greek Olympic contestants, Spartans, Russian Cossacks, and several others.
Later, after being sent into the future, he is taught the ability to jump hundreds of feet into the air by a species of blue gorilla and a jungle man, thus allowing him to reach vast areas he previously could not reach while also giving the impression that he can fly. Jack's magic sword was forged by the gods Odin, Ra, and Rama through three mortal avatars. The sword was forged from the righteous energy within Jack's father; it is able to cut through all but the most magically-protected targets. On the other hand, it is unable to harm beings that are pure of heart, where Aku steals the sword and attempts to kill Jack with it, but fails even to cut him.
Jack strongly exhibits the characteristics of a stoic hero. He is unfailingly polite and humble despite the completely alien nature of the futuristic world and never scoffs at or disparages the customs of the people he encounters (as unpleasant as they seem to him at times). Despite his almost hopeless situation, he does not bewail his destiny, instead exhibiting a strong amor fati. Jack consistently shows an uncommon moral strength of character by helping the poor and defenseless along the way, in one instance even helping talking dogs that worked for Aku, in another, releasing the souls of a family in a haunted mansion. Occasionally, he faces great physical pain, or has to forget his own goals in order to help someone in need.
he began using the name Jack when three teenage aliens, after witnessing Jack survive a huge fall by jumping onto cars, referred to him as Jack while praising him when he landed - in this case, more of a generic term, à la "dude" or "guy." Later, when asked to identify himself, he replied "They call me Jack."