
If he stayed human, Ono would age, die, and reincarnate without him. If he turns human, he realigns themselves together where they'll die, reincarnate, and continue to be fated to meet in subsequent lives (if they are indeed fated, hence trusting in the future). Hisa gradually falls for Ono before discovering that Ono is the past boy but consciously denies it because of his devotion/guilt. Generally speaking, this was a very Japanese-Buddhist storyline.

Hisa (Enma) didn't start loving Ono when he realized he was the reincarnation of Hayo, rather, Hisa loved Hayo and felt that loving Ono would be a betrayal of his love for Hayo. Once Hisa learns that they are the same "soul", he knows that it is okay to love Ono because he is Hayo. For those who believe in reincarnation - which I do - the soul is eternal and is fated to love, or be tied to, only one other soul for eternity. Some stories tell of each soul being split in two at the moment of creation, others talk of two souls being joined and then split later. Basically it is the story of our compulsion to find our "other half" or the "one who completes us". If it is something you have never experienced, it's hard to describe - like describing colors to a person who was born blind - but when a soul touches their "other half" the feelings are different than just "love". When it happens, you just "know". Yes, I am speaking from experience.

Oh, that sounds interesting.
How do you find that person? Is it fate or there could be a possibility of not finding them?

If you're speaking in terms of individual human life spans, it is possible that the soul will be kept apart from their other half - consider the great number of people who die never having found "true love" or happiness with another person. Take heart though, eternity is a very long time and our life spans are exceptionally short in comparison. So, although we may be parted, it is only for a short time. Fate and circumstance are what guide us to find our other half. There is no set way to seek your other half because you won't know what to look for until you find them. The soul doesn't have a look or a personality type that can be described. Our experiences shape our personalities and our bodily DNA shape our appearance. These things don't effect the soul. So, until you stumble upon your other half, you'll be oblivious to them. The kicker is, you may not like the person who currently houses your other half, but you'll be drawn to them regardless. This usually prevents the two halves connecting or they attempt connection, it ends poorly for the people, but they never "get over" the feelings for the other person.
Please understand, I have tried to give an explanation and description of something that is outside the realm of current human understanding. Much like a person who has experienced a religious Epiphany or paranormal phenomena, I am limited in analogies and descriptions that can be related to anyone who has never experienced the same situation. I can't prove what I have experienced and I can barely describe it accurately, but I know that I experienced it. To me, my experience was special and beautiful and I hope that everyone else can experience the same special beauty. Whether they attribute the special beauty to the same cause as I have is not as important as them experiencing it for themselves.
I decided I did not like this story because of all the reincarnation. I don't under stand why he couldn't just stay as a demon in hell. It's strange that he only decided to love the uke when he realized he was the reincarnation of the little boy that his mom killed. Because of that it turned me away from the story. Why couldn't they have just been fated together? It just wasn't to my taste.