an awesome (in my opinion) manga that revolves around Jio Freed and his so-close-to-being girlfriend Ruby Crescent and some random kid he picks up somewhere named Ball. the plot begins when Jio picks up Ruby while checking out some ancient ruins(hes a little kid, bout 9-10 years old at this time, mind you)(Ruby is about 14-15) when suddenly they are attacked by some underlings sent by a guy who claims to be Satan who is trying to get to Ruby because he is looking for o-parts(mythical objects that react when a person uses their soul on them) and she is incredibly good at translating the writing on the ruins. Jio fights him and nearly dies, but reveals himself to be the REAL Satan, or at least the real satan is sealed in his left hand. he then becomes ruby's bodyguard and helps her in HER quest for o-parts. it gets EXTREMELY complicated after that, but its still awesome.
666 Satan is a great manga.