Homare Koiwai is a colleague of Yuta Sakurai at their workplace, and he previously worked at the same company as the protagonist, Moriko Morioka, though they were not directly colleagues at that time. His connection to Moriko from her past professional life places him in a unique position, as he is aware of the person she was before she left the workforce.

In terms of personality, Koiwai is characterized as a bright and sociable mood-maker with high communication skills, allowing him to blend into any situation. He presents a stark contrast to the more reserved and earnest Yuta; Koiwai is generally more easygoing and assertive, while his colleague is more responsible and cautious. This dynamic often leads to Koiwai taking a teasing and playful approach in his interactions, particularly with Yuta, whom he affectionately nicknames Princess Sakurai. He seems to enjoy toying with those around him, but this behavior is undercut by a surprising devotion to the well-being of others, as his fundamental belief is in preventing people from suffering. While some viewers find his snarky and meddlesome nature an entertaining catalyst for the plot, others have found his constant interference to be a frustrating distraction from the main characters' relationship.

Koiwai’s primary motivation in the story is to act as a facilitator, or a shipper on deck, for the romance between Moriko and Yuta. His actions are at least partially driven by a respect for the hard work Moriko performed before she quit her job, and he seems interested in helping her find happiness. He function is largely that of a plot device, pushing the two leads together and orchestrating situations that force them to confront their feelings for one another. For example, his interactions and comments about Moriko provide Yuta with valuable glimpses into her past unhappiness at work, helping Yuta understand her better. Koiwai also takes Moriko out for drinks, a situation that leads to a series of chance encounters that propel the central relationship forward.

His key relationships are predominantly with the two main characters. With Yuta, he shares an office-buddy dynamic; they are coworkers who get along, but their friendship seems largely rooted in their shared professional environment. Koiwai often uses his outings with Yuta as an opportunity to provoke him and give him advice, albeit in a roundabout and teasing way, hoping to push the hesitant Yuta to take action. With Moriko, he reconnects after she has left their former company. He shows a genuine interest in her as a person, but his interactions with her often serve a dual purpose, as he also uses their time together to talk about his friend Yuta, working to build a bridge between the two. Many observers note that his overtures toward Moriko are not a serious romantic triangle but rather a calculated strategy to make his friend jealous and motivate him.

Koiwai does not undergo significant personal development or change throughout the series, as his role is more functional than transformative. He remains a consistent and stable presence, using his social savvy and energetic personality to gently manipulate circumstances for the benefit of the more introverted protagonists. He is portrayed as someone who is already confident and comfortable in his own skin, needing no personal journey of recovery. His notable ability lies in his expert social engineering; he possesses high-level interpersonal skills that allow him to read situations, understand the underlying feelings of the people around him, and create opportunities for connections to form, making him the primary catalyst for the plot’s progress in the real world.