Sōsetsu used to tell his daughter about the beauty of the spring season, and that she would one day see it, in the Land of Snow, where it was forever winter. To this end, he created a generator to warm the climate and when it is done the Spring will come in the Land of Snow. This generator is considered to be the Kazahana treasure, and is activated by a hexagonal crystal, which is considered the key.
Sōsetsu lived in the beautiful Kazahana Castle until his brother, Dotō Kazahana, burnt it down, which killed everyone inside, and forcefully took control over the Land of Snow and for the treasure (which Dotō had no idea what it was), leaving Koyuki the only survivor and escaping with the hexagonal crystal. Sōsetsu's death would scar Koyuki for then next ten years, leading her to become cold and uncaring, putting emotions forward only as an act, as well as referring his dreams about spring to be a lie, all until the generator was activated to create the spring season, much to Koyuki's delight and Dotō's fury (the latter was angry at the treasure being a mere generator). Sōsetsu's death led Sandayū Asama to develop hatred towards Dotō, and tried to get revenge against the latter but unfortunately ultimately fail.