the promise
Eichi can be referred to as either Mitsuki's first love or perhaps simply a close friend, though I would personally prefer the former.
He is the one she made her promise with. There is little known about him, except that Mitsuki cares deeply for him, going to extreme length in ensure his happiness.
He has a deep interest in space and hopes to become an astronaut one day. He left Mitsuki to fly to America to persue his dreams.
But unknown to Mitsuki, Eichi actually died when his plane crashed on his way to America. Takuto was there to take Eichi's soul, but Eichi refuses to move on to the after life and instead chose to stay as a spirit with Mitsuki. It was for this result she was able to see Shingamis. Though at the end of the series, Eichi can finally see Mitsuki is happy and departs for the afterlife.