Him, the alien. She is a person transformed from bamboo. He waters it, and she repays him, protecting him.
He is immortal. She is not. During a war, she... Died, and the arrow she blocked for him was deeply remembered by him.
Every time he comes to reincarnation, he looks for her past life. But no matter how hard I searched, I couldn't find it.
Finally in modern times, he is the teacher and she is the student. He is determined to protect her.