Piano...
Inspired by finally getting to watch the series all the way through.
It was that piano, of all things, that made him realize it. They had found it in Mechano City, just before the revelations of the Great War were revealed.
"What's the matter, Mushra? Don't you like it?"
"It's the most beautiful thing I've ever heard!"
That was when it clicked. Everything about Yakumo was beautiful. Her smile, her face, her hair, her body. Everything. She was everything to him. Sago and Kutal would go to great lengths for Yakumo, even die for her.
But he... he would live for her.
As the piano keys rang out, weaving a tapestry for the ears, he knew that, no matter the foe, he would fight, if Yakumo willed it. She had saved him once before, when he was hanging from that waterfall. Most would say that Mushra had paid her back with interest, but he would never agree. She had provided far too much happiness, just by smiling, for him to ever pay her back.
Some say that he was foolish and stupid, and that everything he did was a result of that. They were partly right, to be honest, but he didn't charge recklessly into battle for battle's sake. It was always for Yakumo. If she was safe, even if he felt like dying, then he was happy.
He wasn't her guardian because he was interested in Shinzo, or whatever wonders laid buried there.
He was her guardian because he loved her.