Hideaki Yoshinari is a supporting character in the visual novel Collar × Malice and its two-part anime film adaptation, Gekijōban Collar×Malice Deep Cover. He serves as a junior officer in the Special Police (SP) division and is the subordinate and partner of fellow SP officer Kei Okazaki. The unit is assigned to observe and report on the activities of a former detective named Aiji Yanagi and his team as they investigate the terrorist attacks known as the X-Day Incident.

In terms of background, Yoshinari is a young man with a bright and energetic demeanor. He is described as having blond hair and hazel eyes. His personality is defined by his friendly and talkative nature, but he has a notable habit of speaking whatever crosses his mind without thinking it through first. This blunt and outspoken tendency often becomes a source of annoyance for the other characters, leading to his senior partner, Okazaki, swiftly punishing him. This dynamic is a running gag within the story. Despite this clumsiness in social situations, he is recognized as a capable and loyal officer who is entrusted with important tasks. For instance, he is often charged with ensuring the safety of protagonist Ichika Hoshino and her younger brother Kazuki when Okazaki is unavailable. His loyalty is further demonstrated through his deep concern for his superior, Okazaki.

Yoshinari's primary role in the narrative is as a supporting member of the police force assisting the main investigation. His interactions are centered almost entirely on his relationship with Kei Okazaki. As Okazaki’s junior, he is frequently the target of his partner's exasperation. However, this relationship becomes more significant and heartfelt in the aftermath of the main incident. In the fan disc Collar × Malice: Unlimited, Yoshinari plays a crucial role in Okazaki's character development. Devastated by the "tragic love ending" of the original game, Okazaki becomes withdrawn and loses his will to live. Yoshinari persistently visits his partner, charging his phone, bringing him food, and ultimately slapping him to snap him out of his self-destructive despair. He forcefully reminds Okazaki that he cannot simply throw his life away, showing a maturity and depth of care that contrasts with his usually clumsy exterior. This act is instrumental in helping Okazaki begin to heal.

Regarding his abilities, Yoshinari is a trained SP officer, which makes him proficient in protection details and the use of a handgun. While he is noted for having keen observational skills, this strength is frequently undermined by his inability to keep his observations to himself, turning his insight into a social liability. He is described as having a "butt monkey" role in the series, often being the subject of teasing due to his behavior, but he is ultimately a reliable and kind-hearted ally.