In the old times were angels walking among the humans and were helping them. And among them were angels, called Yuki Tenshi. Snow Angels. They came to Earth with the first snow and with snow they disappeared. They only concern was to make people happy. They had the power to make them smile even in the cold of winter, when most humans were falling into melancholy and depression.
One of those angels was a young girl. Unfortunately she fell in love with a human, which was forbidden. She was immortal angel and he was only human with limited life span. And because she was Yuki Tenshi, she had to leave every spring. The goodbye was always very hard and painful, but the home coming was even more joyous. Their love didn't weaken even after long years. But the man was aging. Every year he was older and older and in the end he was left old man, while the girl's face was un-touched by the powers of time. The old man knew that his time was nearing.
As he was dying, the girl was holding him in her arms. Her sobs were heard far away and her tears were streaming down her cheek and falling down on the ground in the form of crystal clear jewel. The old man took his last breath. The girl was crying bitterly, then she lied his beloved down and she lied down beside him. Then she let appear her wings, even thought it was forbidden in the human world, and cowered them both. She was prying, that no other angel would go through what she was going through. She died from broken heart beside her love and through her sacrifice she unconsciously created a way for other angels. She created a way for them to change into humans.