Yankee Quest

The strongest gang in the Kanto region is the Satans, by far, who have taken all the other gangs in the region under their command. The reason for their dominance is none other than their leader Yamaguchi Nitaka who is bar none the strongest fighter. With Nitaka taking charge, they begin their campaign on the Kansai region and all is going well until a flash of light takes Nitaka and his friend Mizuno Tsukasa to another world.
When the pair come to, they are greeted by a princess who tells them that they are in a world called Olicia. In this land, magic, demons and mythical creatures are not just fantasy or a videogame but absolutely real. The princess's kingdom has been under siege by the Demon Empire and as a last resort, she attempted to summon Satan himself to defend her people. Although she mistakenly brought Nitaka over instead of the lord of the underworld, Nitaka's strength knows no bounds and fights equally as well against dragons and wizards.
(Source: MangaHelpers)