Jubei is the main character of Ninja Scroll. He is a wandering Warrior, known as a Ronin which is a masterless samurai/ ninja. He makes a living doing odd jobs while traveling. As Ninja Scroll starts he is retrieving a sword that is worth 300 gold pieces for a poor village (for only 20 gold pieces).
Jubei is a very intelligent and calculating person. He has very sharp wits and is a master of his sword technique. He's basically a perfectly well rounded warrior. No matter what the odds, Jubei will always fight with all of his heart if it's something he believes in (whether that be protecting Kagero or sending Gemma back to hell).
Because of Jubei, Kagero learns that she is a person and even to begins to care for him and herself; something that she had never done before. He falls in love with Kagero... and even though he is poisoned and going to die, he will not treat Kagero as something to be used for personal gain. He would rather die than treat her that way, and accepts his fate unknowing to him that he has been cured.