Kikou Majutsushi: enchanter

Fulcanelli is an "Enchanter," an ancient engineer/alchemist who is able to imbue the weapons and gadgets he constructs with magical powers. Transformed into a demonic being after his death, he is always accompanied by his demon lover Eukanaria. Due to circumstances still unknown, he is in need of a new body. Until one can be found, Eukanaria must guard the pendant containing his soul from the many demonic forces that wish to obtain his great power for their own.
In the present day, Haruhiko Kanou is an ordinary boy good with machines and nursing a crush on his science teacher and neighbor Yuki Fujiwara. Little does he know he is Fulcanelli's perfect double and Eukanaria has tracked him down...

(Source: DMP)