Mugen (??) was born in a penal colony on the Ryukyu Islands, which in the Edo period was a separate kingdom with close ties to the Satsuma domain of Japan. His name means without limitation, as the components of it are the characters for "without" and "limitation." Incidentally, he writes his name using the infinity symbol (?), a pun on his name, 'Mugen,' which is a homophone of the Japanese word for 'eternity'. His Japanese is quite rough and "Yanki"; and he is illiterate.

The exact date of his birth is unknown, and much of his past is shrouded in mystery. It also appears that he may be of Ainu origin. The blue tattoos on his wrists and ankles indicate that he was once in prison. The circumstances of his imprisonment are unclear at first, but it is later verified that he was to be executed for engaging in piracy.

Rude, lewd, vulgar, and nihilistic, Mugen is somewhat of an antihero.

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