Shigehito Hasumi is a successful journalist and a supporting character who serves as a primary romantic interest for the protagonist. He shares a history with Sumire Iwaya, having attended the same university, where he was her senior and first love. As an adult, he is an ace journalist working for the foreign news desk of a major newspaper, having returned to the headquarters from a bureau in Jakarta. He is described as the type of man who embodies the three highs tall, high income, and well-educated, making him a figure who fits the traditional ideal of a desirable partner.

In terms of personality, Hasumi is presented as a composed and accomplished professional who outwardly represents the perfect man. Despite this image, he is not without flaws and is known to be prone to making silly mistakes, which adds a layer of vulnerability to his character. However, his presence often creates a sense of formality. Sumire finds that when she is with him, she reverts to being the insecure and shy woman she was in her youth, unable to express her true feelings or be completely at ease. He is aware that Sumire is not entirely relaxed around him and makes an effort to encourage her to show her genuine personality.

Motivated by a long-standing affection, Hasumi wasted no time in proposing a serious relationship with Sumire after they crossed paths again as colleagues. His goal is to build a life with her, and he desires to marry her. This intent to form a traditional, committed relationship represents a key path for Sumire, one that aligns with societal expectations of a successful career woman. This aspiration is consistently thwarted by several factors, including their demanding careers, which lead to a transfer for him, the manipulative efforts of a rival colleague who wants him for herself, and the unusual living situation Sumire hides involving her pet.

Within the story, Hasumi functions as the central obstacle to the unconventional bond between Sumire and Momo. His presence as the perfect on-paper boyfriend contrasts sharply with the honest and comfortable relationship Sumire shares with her pet. While the relationship with Momo allows Sumire to be her true self, her dynamic with Hasumi is built on pretense and mutual admiration of an ideal. He is largely unaware of Momo true nature as a man, believing him to be an actual dog. As the story progresses, it becomes increasingly clear that despite his admirable qualities, he is not the right partner for Sumire because she cannot achieve the same level of comfort or be her authentic self with him as she can with Momo.

Key relationships define his role. His connection with Sumire is complex, rooted in nostalgia and the pursuit of an ideal, yet ultimately lacking genuine intimacy and mutual understanding. He also attracts the aggressive attention of a female coworker who uses her appeal to try and win him over. There is little notable development for Hasumi as an individual throughout the story, as his primary function is to highlight the personal growth of Sumire and the authenticity of her connection with Momo. His notable abilities lie in his professional competence as a top journalist and his embodiment of social status and success, which are the very qualities that make him a seemingly perfect, yet ultimately unsuitable, match for the heroine.