He is Han Zhen, From the kingdom in the past when trapped in the forest behind the fireworks kingdom. Han Zhen saw an illusory light and accidentally entered a modern world full of buildings, but now he is in the middle of the street wearing a prince's clothes.

some time passed, Han Zhen was adopted by a family from the city. His status as a prince was kept a secret and now his life begins as Hanjin, a girl who becomes your little sister at home because you are adopted. The girl is the same age as you.