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]

You play as Azai Kyousuke, the son of an infamous gangster. Kyousuke knows what he likes and lets nothing stand in his way in getting what he wants. Referred to as God by his classmates, he often enjoys listening to Bach and "working" part time for his stepfather. But when a beautiful girl named Usami Haru appears in town along with an international criminal known as "Maou," they bring with them their cat-and-mouse game of plotting and intrigue.