Opera Kobayashi is the central character of Tantei Opera Milky Holmes Alternative TWO and the main protagonist of the original Milky Holmes game series. He first gained fame as a prodigy known as the boy detective Kobayashi Shonen, solving numerous difficult cases at a young age. His active career as a detective ended abruptly several years before the story begins, when he lost his Toys ability during a confrontation with the phantom thief L. Because Toys are the special powers that detectives in this world rely on, losing his left him without confidence and caused him to withdraw from public life. He was later called to the Holmes Detective Academy and appointed as the instructor in charge of the detective team known as Milky Holmes.

In terms of personality, he is ordinarily a calm and mild-mannered young man who can be somewhat easily swept along by events, but when faced with a case he becomes restless and unable to sit still. He is also passionate and articulate, known for his energetic way of speaking and for his habit of declaring that something is an important factor. His name is drawn from Kobayashi Yoshio, the boy detective of Edogawa Ranpo's Boy Detectives Club novels.

In the events of Alternative TWO, he is active in London, where his students from Milky Holmes follow him. Though he no longer possesses a Toy, his observational skills, insight, and knowledge remain far beyond those of an ordinary detective, and even veteran phantom thieves acknowledge his abilities. His role in the story is that of a mentor and lead investigator who directs the team's reasoning and guides the others through a serious criminal case involving new allies and adversaries in London. His main motivation is to see justice carried out and to lead his students through proper detective work using knowledge and teamwork.

His key relationships are with the four members of Milky Holmes, the students he trained and who trust and rely on him deeply. Working alongside them again gradually helps him recover the confidence he lost when he gave up being a detective. The London setting also brings him into contact with figures such as the detective academy student Lily Adler, Scotland Yard inspector Hartley Queen, the phantom thief Kitty Evans, and the military officer Edgar Moran, who act as allies, rivals, or opponents over the course of the case. His most notable ability is not a supernatural power but his detective mind, which allows him to assess situations quickly and lead the group with intellect, experience, and composure. His development in the story traces his path from a man who had lost his place as a detective back to someone who stands at the center of the investigation once again.