Hasunuma is a character who initially appears as a support figure before taking on a central role with significant importance to the plot. From the beginning, he exhibits unusual behavior and seems to possess a deep awareness of the secrets surrounding other characters. Despite his oddities, he functions as a good teacher who is adored and popular with his students. He jokes with them and is known to turn a blind eye to some of their mischievous activities at school.

Personality wise, Hasunuma is depicted as mysterious and highly intelligent. He often operates from the shadows, solving problems when situations become dangerous for the main characters. His sense of justice, though unconventional, is absolute; he personally makes the abusive teacher Aoki sensei literally disappear when the mistreatment of a student comes to light. Later, he shows a similarly protective and threatening side by warning Kurokawa that he would make her disappear if she ever harmed Ikeno through the same kind of abuse she herself endured.

His role in the story evolves significantly as his true nature is revealed. It is eventually disclosed that Hasunuma is not human. He is an alien who made a deal with the real Hasunuma, a man who wished to end his life. The alien borrowed the teacher's body to accomplish his own goals, while the original owner has been sleeping for three years. The alien explains that he came to Earth for a deeply personal reason: to find the person he truly loves, a woman named Hana. In his home world, there are five gender types, with the first type being extremely rare and holding high status. As one of only seven surviving beings of the first gender, he was controlled and protected all his life. Hana, who is of the third gender, was once his caretaker. She was the only person who behaved normally with him, even scolding him when necessary, which caused him to fall in love with her. When she was transferred to Earth, he escaped his world without permission to find her.

Regarding key relationships, Hasunuma pays particular attention to Ikeno, Kurokawa, Tsurumi, and Shijou. He proposes to Kurokawa Rika to become the mother of his child, stating that he likes her. However, he simultaneously admits there is another person he truly loves, which is Hana. Because of his feelings for Hana, he shows more care toward Komaki than he does toward Rika. Tsurumi, who never liked Hasunuma due to his suspicious nature, begins calling him by the nickname "fake Hasunuma" after the truth is revealed.

Hasunuma possesses notable abilities, including special powers similar to the main characters. Specifically, he has all five senses significantly enhanced. As an alien, his nature differs from others on Earth; while some aliens live inside humans in a dormant or awake state, Hasunuma took full control of his host body. In the end, he plans to remove the aliens from the heads of others in order to save them and bring them back to his home.