Nn-ueinu-kf, later referred to as Takopi, is the eponymous main protagonist of Takopi's Original Sin series. He is an alien being, a Happian originally from the Planet Happy, a world centered around spreading happiness and positive emotions. He arrives on Earth in the year 2016 as part of his mission to bring joy to others, equipped with a set of advanced tools known as "Happy Gadgets," designed to solve problems and improve people's lives.

After crash-landing on Earth, he encounters a young human girl named Shizuka Kuze, a withdrawn and deeply troubled child dealing with severe bullying at school and emotional neglect at home. Moved by her evident sadness, Takopi becomes determined to help her feel happiness, deciding to stay with her and support her however he can.

However, Takopi’s understanding of Earth, human emotions, and the complexities of interpersonal relationships is extremely limited, and despite his sincere intentions and gentle nature, his actions often result in unintended harm. Lacking awareness of the long-term consequences of his choices and the depth of human psychological suffering, Takopi's efforts to improve Shizuka's life lead to increasingly serious and tragic complications.