Seif is a Vice Admiral of the Marines who is an accomplished swordsman who specialises in the art of stealth and assassination. He is a master of the Rokushiki technique "Soru". As an underling of White Panther, Seif believes in absolute justice. He will do whatever it takes to ensure the world is rid of all scum, and turned pure once more. Many who oppose the marines see Seif as nothing more than a tool to do White Panther's dirt work, hence the epithet "Mercenary".
Seif is an incredibly cold-hearted, intimidating man. He prefers to stay away from the other marines and only reveals himself when given a mission, much to his soldiers' relief.
As Seif rarely talks to others, not much is known about his past. However it is speculated that he grew up alone, which lead to him developing his cold heart. Many people believe he joined the marines merely so he had a reason to kill and make people feel as lonely and cold as himself.