G-senjou no Maou - The Devil on G-String

You play the role of Azai Kyousuke, the son of a legendary gangster infamous in the underworld. You spend your time listening to Bach, playing God at school and covertly working for your stepfather, a ruthless financial heavyweight. This idyllic existence is broken when two individuals appear in the city - a beautiful girl named Usami Haru with hair you could get lost in for days, and a powerful international gangster known only as "Maou". Almost without delay, the two begin a deadly cat-and-mouse game, bringing you and your friends into the crossfire. Plotting, political intrigue and layer upon layer of interlocking traps are the weapons in this epic battle of wits.

[Edited from the description by Asceai]