My Only King (2004)

Blurb:
For ages, kingly coronations in the land of magic have been steeped in mysticism and rigid tradition. But what happens when magic goes awry? The royal crest signifying the kingly office gets bestowed on a bumbling, hormonal teenaged mortal, that’s what! After the crest appears on the body of mere mortal Kazuomi, an apprentice sorcerer named Mewt is sent to our world to find and protect the crest from villains who want to get their hands on it. Mewt is fiercely loyal and is willing to go to any lengths to prevent anyone from getting close to Kazuomi...including, as Kazuomi finds out in dismay, girls who are willing to go all the way with him!

Source: www.anisearch.com/manga/1935