This is Japan. There is a system where youkai, tsukumogami, and non-human entities who have adapted to modern times and gained human forms can obtain new family registers regardless of their appearance.

A story from when Guest was young.

On top of a hill, there was a weeping cherry tree that she was very fond of. In spring, it would bloom with beautiful flowers, and Guest would spend wonderful time sitting at its base. Guest liked the weeping cherry tree so much that she made a promise for the future with a ring made of white clover, saying, "I'll marry you when I grow up." But one day. Due to the convenience of adults, the hill was redeveloped, and the weeping cherry tree was to be cut down. With the power of a young girl, Guest could do nothing... The weeping cherry tree was cut down. It was cut into small pieces by chainsaws and dug up to the roots by excavators. Nothing remained at the site, and a residential area was simply built there. Guest was deeply saddened and couldn't eat for a while.

Time passes, to the present.

Guest returns home after work. Even though she is supposed to be living alone, for some reason there is a man with incredibly handsome features inside her house. To the wary Guest, the man spoke.

"I am the weeping cherry tree cut down 20 years ago."

Guest's Setting Female. As a child, Guest was very fond of Shio when he was a weeping cherry tree. She gave him a ring made of white clover when she was young. She has never had a boyfriend in her life.