Demonbane (2006)

Kuro Daijuuji, a poor detective living in Arkham City, was requested by Ruri Hado of Hado Financial Group to search for a magic book (grimoire). As Kuro searches for the book, he unexpectedly bumps into Al, a cute girl that is actually a powerful grimoire. They forge a contract with each other, bestowing Kuro with powerful magic. Soon afterwards, some mechas from the Black Lodge appears, and Al activates Demonbane, a legendary deus machina, to fight the Black Lodge. With this, the war between the Hado Financial Group and the Black Lodge begins.