Janus, though they are very much a real and very rare race, are considered myth by the inhabitants of Farther Earth. They are a "theoretical" perfect cross between nymphs and Sylphs that talented magicians seek out, in hopes that they could somehow learn how they can harness all types of magic regardless of whatever race they were born as.
Mephisto is the father of the only two Janus in existence, X and Len. He learned, somehow, that they have to be created with an ancient spell, and be born from a Sylphic mother and nymphid father. He made them for the sole purpose of enacting chaos upon farther earth, as catalysts for upsetting balance between the dimensions.
A prophecy foretold by Faust, who Mephisto trusts as an advisor of sorts, spoke that Mephistopheles's offspring would either be his path to complete control of both realms, or his path to annihilation. He decided to take the risk, but have two, so that he could pay attention to both and destroy the one that would be his downfall. He created the two Janus, a pair of half siblings who would never meet each other, and let them grow on their own. He did not interact with either of their lives, as to not disrupt Faust's prophecy, until his daughter, Len, began to show greater abilities than his son, who had already passed on. Len was training to protect Siverial from the Black Crusaders, who, ironically, were founded by her brother. Mephisto murdered her before she could actually get in the crusader's way. However, Len's murder would be what leads to Mephisto's defeat, as Alex would not have learned how to process grief, and therefore would not have become a suitable conduit for Abaddon's flame.