Mitsuhiko Tsubaraya is a first-grade student at Teitan Elementary School and a founding member of the Detective Boys, an amateur detective group formed after a series of early adventures with Conan Edogawa. He comes from a family of educators: both of his parents are teachers, and he has an older sister, Asami, who is about ten years his senior. His upbringing has instilled in him a strong sense of formality and respect, leading him to consistently address adults and even some peers with polite suffixes.

In personality, Mitsuhiko is intelligent, inquisitive, and methodical. He has a deep interest in science, history, and general knowledge, often reading books on these subjects for personal enjoyment. He approaches problems with a rational, scientific mindset and believes that technology and careful reasoning can solve most obstacles. This makes him the most logically minded member of the Detective Boys, frequently serving as the group's strategist and voice of reason when Conan or Haibara are not present. He is also highly proactive and enjoys being prepared for any situation, as seen when he brings multiple flashlights and a compass on their first outing or takes detailed notes during investigations in case they prove useful later. Despite his maturity, he still enjoys age-appropriate activities such as treasure hunting, building snowmen, and watching action shows like Kamen Yaiba. He has a known dislike for carrots and will avoid eating them whenever possible.

Mitsuhiko's primary motivation is the pursuit of knowledge and the desire to be useful. He takes pride in being able to contribute facts or insights that help solve cases, and he feels rewarded when his friends acknowledge his helpfulness. He is also driven by a genuine care for those around him, which sometimes leads him to take personal risks to please or protect his companions. Over the course of the story, he develops romantic feelings for both Ayumi Yoshida and Ai Haibara, though he is initially uncertain about how to handle these emotions and seeks advice from older friends about whether it is acceptable to like two people at once.

Within the narrative, Mitsuhiko plays a key supporting role in the Detective Boys. His ability to think ahead and recall factual information often fills gaps when Conan or Haibara are unavailable. He has been shown leading the group in escape scenarios, deciphering clues, and even caring for an injured Haibara by using a bandage technique Conan taught him. While he is not as brilliant as Conan or Haibara, his steady curiosity and diligence make him a reliable asset in investigations. His relationships with his teammates are close; he has been friends with Genta and Ayumi since preschool or kindergarten, and he develops a particular fondness for Haibara after an incident where he carries her to safety. He respects Conan and often learns from him, and he addresses adults such as Professor Agasa and police detectives with formal respect.

Mitsuhiko undergoes gradual development throughout the series. Initially somewhat hesitant and reliant on others, he grows into a more confident and independent thinker. He learns to take initiative, as demonstrated when he ventures alone into the mountains to catch fireflies for Ayumi and Haibara, and when he steps up to solve riddles or direct the group during critical moments. His deductive reasoning skills improve steadily through exposure to real crimes, and he becomes more adept at noticing details and thinking logically under pressure.

Notable abilities include his strong memory for facts, his proficiency in applying scientific and historical knowledge to practical situations, and his habit of careful note-taking. He is skilled at using the Detective Boys' gadgets, such as the detective badge and radar glasses, and he knows basic first aid techniques. His analytical approach allows him to break down complex problems into manageable steps, and he is often the one to point out inconsistencies or overlooked evidence. While he can sometimes misjudge danger or become lost in his own pursuits, his overall resourcefulness and dedication to his friends make him an effective junior detective.