Happōsei is a central character in the planetarium film Nintama Rantarō no Uchū Daibōken with Cosmic Front☆Next. He is a transformed version of the character Happōsai Hieta, the ambitious and scheming headmaster of the Dokutake Ninjutsu Classroom and leader of the Dokutake Ninja Team. In the main Nintama Rantarō series, Happōsai is known as a longstanding enemy of the Ninjutsu Academy, frequently plotting elaborate schemes against the school and its headmaster, though his plans almost always fail in comedic fashion. He is easily recognized by his unusually large head, which often causes him to tip over backward when he bursts into his signature villainous laugh. A running gag in his appearances involves him becoming irritated when others mispronounce his name, leading him to loudly correct them.

The story of the planetarium special begins one night as the young ninja trainees Rantarō, Kirimaru, and Shinbei are stargazing. Happōsai arrives on the scene with the intention of stealing a swan-shaped paddle boat from the Ninjutsu Academy. However, before he can carry out his theft, a shooting star unexpectedly plummets from the sky and strikes him directly. The impact of this cosmic event triggers a dramatic change in his personality and knowledge. The scheming, war-enthusiast villain is replaced by a completely different persona: Happōsei, an individual who is suddenly and passionately knowledgeable about space and astronomy. The stolen paddle boat is also magically transformed into a spaceship, and Happōsei enthusiastically persuades the three hesitant ninja trainees to join him on an impromptu journey into the cosmos.

As Happōsei, his former motivations for conquering the Ninjutsu Academy are completely forgotten. His primary drive becomes a boundless enthusiasm for space exploration and a desire to share astronomical wonders with the boys. He effectively serves as the group's impromptu guide and expert on their adventure, leading them through the solar system and beyond. His role shifts from that of a comedic antagonist to an eccentric, if somewhat unreliable, mentor figure on a scientific expedition. Throughout the specials, which are structured as educational collaborations with the science documentary series Cosmic Front☆Next, Happōsei is the character who facilitates the learning experience, explaining various celestial phenomena to Rantarō and his friends as they encounter them. His key relationships are with the three main protagonists, Rantarō, Kirimaru, and Shinbei, as they are forced to work together with their former enemy in the unfamiliar environment of space. The dynamic is one of mutual bewilderment and curiosity, with the boys initially suspicious of the transformed Happōsei but gradually drawn into his excitement about the universe.

Regarding character development, Happōsei represents a radical, albeit temporary, departure from the established persona of Happōsai. His transformation is not a gradual growth but a sudden, magical change instigated by the meteor strike. This event strips away his cunning and malicious traits, replacing them with a genuine, almost childlike wonder for astronomy. His notable abilities are a direct result of this transformation. While the original Happōsai is a skilled ninja and strategist, Happōsei possesses an encyclopedic knowledge of space. He can identify planets, explain the properties of the moon and stars, and navigate the dangers of the cosmos, such as black holes. This educational role is the core function of the character within the special, turning a classic villain into an unlikely and enthusiastic science teacher for a journey through the universe.