Hayato Izumi is the male protagonist of the series, a high school freshman at Seifu High School who serves as the vice president of the student council. He lives alone in an apartment, as his mother is deceased and his father works abroad, traveling the world. Hayato is described as a serious, studious, and pragmatic individual who initially ran for student council president with conventional policies. He lost the election to Ui Wakana, whose charismatic and provocative campaign promise to "liberate love" on campus, which included distributing condoms, proved more popular.

Following his defeat, Hayato is appointed as the student council's vice president. His life takes an unexpected turn when Ui arrives at his apartment, revealing that their parents made a drunken agreement years prior that arranged their marriage, forcing them to live together. Initially, Hayato is resistant to this arrangement and to Ui's overtly romantic and sexual advances, struggling to maintain his serious demeanor while hiding their cohabitation from the school. He is often driven to extremes to keep their secret, such as when he falsely claims that Ui's underwear and lip gloss are his own, inventing an embarrassing cross-dressing hobby as a cover story.

Hayato's personality is defined by his role as the "straight man" to the energetic and unconventional Ui, frequently providing a common-sense counterpoint to her actions. Despite his protests and attempts to resist temptation, he is not insensitive. He experiences physical reactions to Ui's intimate provocations, such as nosebleeds and visible arousal, which manifest internally as a "Bad Angel" that tempts him. He is self-sufficient and accustomed to managing all household chores, a trait that contrasts sharply with Ui's dependent and clumsy nature at home. While he often feels uncomfortable with intimate acts like kissing, once engaged, he can become more willing and even somewhat sadistic.

Hayato's primary motivation throughout the story is to navigate the chaotic situation of his secret engagement and cohabitation with Ui while fulfilling his duties on the student council. His key relationships drive much of the plot. His relationship with Ui Wakana evolves from reluctant acceptance to genuine respect and romantic affection, culminating in a public marriage proposal at the school. He also has a significant and complex relationship with Rin Misumi, the stern head of the school's disciplinary committee who opposes Ui's liberal policies. Rin is also Hayato's childhood friend, and their relationship begins antagonistically due to their opposing views, but it later develops into a source of belligerent sexual tension and a love triangle. Hayato develops a notable fixation on Rin's large breasts, which adds to the conflict with Ui. His interactions extend to other characters like Kei, Rin's cross-dressing sister who serves as the school nurse and helps Hayato with cover stories, and other members of the student council.

Throughout the series, Hayato undergoes significant character development. He transitions from a reluctant fiancé who is frequently flustered and opposed to the situation into a more accepting and committed partner. A memory of a childhood promise where Ui expressed her desire to marry him helps solidify their bond. While he dithers between his growing feelings for Ui and his physical fascination with Rin, his arc concludes with a decisive choice to fully commit to Ui, offering her a ring and a public proposal. In terms of notable abilities, besides his self-sufficiency in cooking and cleaning, Hayato's main talent is his quick thinking under pressure, particularly in fabricating elaborate lies to hide the evidence of Ui living with him, such as the cross-dressing alibi.