Izumi Hayato is the male protagonist of the series. He is a high school student who holds the position of vice president of the student council at Seifu High School. His situation is unusual, as he lost the student council presidential election to Ui Wakana, who ran on a platform of liberalizing romance that famously included distributing condoms during her speech. Following his defeat, Hayato accepted the role of vice president, working alongside his new superior.

His life becomes significantly more complicated when Ui appears at his apartment with an arrangement made by their parents while drunk: the two of them are to be wed. This engagement forces Ui to move into his home, a secret the pair must keep from their classmates and the school authorities. Hayato lives alone, as his father works abroad and his mother is deceased, making him self-sufficient and accustomed to managing household chores like cooking and cleaning long before Ui arrived.

In terms of personality, Hayato is depicted as a serious, responsible, and pragmatic individual. He often serves as the straight man, providing a rational counterpoint to the eccentric and often sexually charged situations that arise around him. Initially, he is quite resistant to the idea of the arranged marriage and is visibly uncomfortable with Ui's persistent and forward romantic advances. Despite his verbal protests and attempts to maintain his composure, his body frequently betrays him, reacting with physiological responses such as nosebleeds or visible arousal when faced with intimate provocations. This internal conflict is sometimes visualized through metaphors like a "god of disaster" or "Bad Angel" appearing when his impulses intensify.

Hayato's role in the story is as the central figure around whom the romantic and comedic conflicts revolve. His primary motivation is to manage the dual life of a student council officer and a secret fiancé. He struggles to balance his duties with hiding his cohabitation with Ui, particularly from the strict disciplinary committee led by Rin Misumi. His attempts at concealment lead to embarrassing situations, such as claiming that Ui's underwear and lip gloss belong to him, thereby fabricating a cross-dressing habit to explain their presence in his room.

His key relationships are central to his development. With Ui Wakana, his relationship evolves from reluctant acceptance to genuine respect and affection. Although he gets caught up in her clumsy attempts at seduction, he develops a deeper bond with her, culminating in a public marriage proposal at the school's graduation ceremony. His interactions with Rin Misumi are more antagonistic, beginning with conflict due to their opposing views on school rules. However, this tension evolves as Rin develops feelings for him, creating a love triangle. Hayato finds himself attracted to Rin, particularly noting her figure, which adds another layer of complexity to his commitment to Ui. He also interacts with other female characters, including members of the student council like Rin's sister Kei, the school nurse, but his primary focus remains on the two main rivals.

Throughout the series, Hayato undergoes significant character development. He transitions from a flustered and resistant fiancé to a more accepting and proactive partner, culminating in his declaration of love and a proposal to Ui. He navigates the temptations presented by Rin and others, ultimately reaffirming his choice to be with Ui. Aside from his self-sufficiency and household management skills, Hayato does not possess any notable supernatural or exceptional physical abilities, with his role being grounded in the everyday realities and comedic mishaps of high school life.