An immortal man searching for the source of his immortality to become a mortal again. If seen this concept before in quiet some anime/manga. Usually these people are also accompanied by some legendary swordsman. Is this based on some old myth/legend or on something else?
It seems that several of the other characters and key story points are based upon Greek mythology. All though this has not been confirmed by the author himself yet.
There is no actual figure in Greek mythology called Edermask/Ethermask/Nenomius, the antagonist Janus does seem to have a Greek counter part. janus, the god of beginnings and passages. Is said to be a 2 faced god. One facing the future, and one facing the past.
This god would fit the image of both the characters as their looks are identical, and behaviour matching that of this particular god. One can not let go of the past, and one only wishes to move forward (very, very roughly transcribed).