Kimi Sae mo Ai no Kusari

From ShoujoMagic:

Shouta Koishikawa and Miono Hasuoka, childhood friends and neighbors, have always been together. At some point, Shouta's heart began to recognize only Miono, but it was around this time that Miono began to be charmed by her science teacher, Professor Kaorumaru. Not entirely what he seems to be, Kaorumaru predicts an immediate future in which Miono's fated love will "awaken another Miono." Later that day, as Shouta attempts to finally confess his love... the demon Ginyasha appears and takes control of Shouta's body!! -- awakening something inside Miono! So begins Shouta's struggle to control both the demon and the future. Everything now spins on a wheel of Fate that was set in motion hundreds of years ago, and it's about to come full circle!