To Answer Your Question
"When you can live forever, what do you live for?" Enno Ozuno had asked him once. Zenki hadn't an answer for him then. But now, he knew what he lived for. It was curled up in the bed before him, sleeping at that very moment. He lived for Chiaki because without Chiaki, there was no living. Oh, he'd survive if the girl was to die but it wouldn't be a true life. There would be no happiness or joy. No laughter or light. No love. There would be only surviving, pain, and sorrow.
The moment Chiaki died, he'd stop truly living at all.
Karuma had thought him crazy, loving a mere human, but she didn't understand. None of them did. How could they? They saw only what they wanted to see. The little imperfections and mistakes. They saw only her weaknesses and failures. They didn't care to see her perfections or when she did something right. Nor did they see her strengths and victories. They couldn't look past her human nature to what lay hidden deep down within.
Zenki ran a large, gentle hand down Chiaki's side, being careful of the bone-deep bruises that were hidden underneath the girl's clothes. Her body was covered in large, black and blue spots from where a Karuma Beast had thrown her against the building she had been guarding.
The Karuma had gotten her badly, but luck had been with them and Zenki had gotten the monster back just as well. The mission itself had been a complete success, despite the fact that Chiaki had been beaten black and blue. Nothing was broken thankfully. Ruby eyes watched the steady rise and fall of Chiaki's chest. He could watch her breathe for hours and never get tired of observing the simple movement. Whatever Goki had given her to help dull the pain was working well.
Then again, as Chiaki often said "she'd been hurt worse". He didn't know how he'd ever existed without this little human by his side.
The next morning she'd be up at first light as usual, go about her chores, and by the time dinner came around, it was likely that she would be out on another mission, despite her bruised body. Chiaki often felt that she had something to prove to others, as well as herself. Despite missions and time not spent together, they were happy and whole, for the most part. No one could understand Chiaki better than him.
Well, almost no-one, but Chiaki's granny didn't really count, because she'd had around sixteen more years and raising the girl to go on. The thought of Chiaki's granny made him cringe. He hoped to Buddha that all of Chiaki's bruises were healed by the time she came back or else he was going to get it.
And despite what most of the others thought, there was nothing wrong with running away from a pissed-off Granny Enno. That little old lady had the ability to strike fear into the bravest of them with a glare and a broom, but she loved her granddaughter more than anything, and only wanted to see her happy.
He wanted Chiaki happy too, and it didn't matter what it took for her to get that way. The sight of her smile was enough to send his heart fluttering and the sound of one happy laugh was enough to make him happy for days.
Chiaki was everything to him. His complete reason for being.
The reason he was alive and kicking this very day was because of the human asleep in the bed. Perhaps the reason she tried so hard was to prove to the others that she had earned the right to be with him, and she was only trying to get those who couldn't see that to notice. It was an uphill battle for acceptance, and she was fighting. At times, it seemed as if it was a losing battle, but at the end of the day, all that mattered was Chiaki.
"When you can live forever, what do you live for?"
He had the answer, had had it for a while now.
He lived for Chiaki. For the girl that tried so hard to prove to others that she had earned the love of a demon. He lived for the days they could spend in peace, the battle that he could fight with her near, and the future that he knew would come. He lived for Chiaki and the future that she promised, with three simple words that were spoken a million times a day, even though most of those times were silent.
I love you.