As a child, Ishimaru lived in a small neighborhood with his mother and grandmother. He befriended a devil girl his age, Misaki Takenami, who lived next door. Misaki was skilled at origami, often sharing her creations with Ishimaru and encouraging his own attempts. Despite their friendship, Ishimaru’s attention was drawn more toward Misaki’s mother, Shizuku Takenami. His mother, however, disapproved of his frequent visits to the Takenami household, as local rumors accused the family of carrying a disease, being strange, and exerting a bad influence on neighborhood children.

One night, Ishimaru secretly visited the Takenami home, where he saw Shizuku for the first time without makeup, revealing pronounced dark circles under her eyes. When he asked if she was ill, she reassured him that she was fine, explaining it was “normal” for her. Relieved, Ishimaru was content simply to know she was all right. His visit was cut short when his mother pulled him away, but the encounter made him realize his feelings for Shizuku.

Not long afterward, the Takenamis moved away as the spreading rumors became intolerable. Before leaving, Misaki gave Ishimaru a handmade origami bird as a parting gift. His final memory of Shizuku was seeing her in a car, wearing protective glasses designed to shield against blood.

These early experiences left a lasting impression on Ishimaru, sparking a lifelong interest in devils. He began researching them extensively, including writing essays on the subject, which gave him an unusually broad knowledge of their nature and behavior.

Ishimaru is an adult man with a slender, seemingly frail build, though he is in fact highly capable physically. He is of average height and has brown, drooping eyes with prominent dark circles beneath them. His black, wavy hair reaches his shoulders, with middle-parted bangs that frame most of his face. On occasion, he ties his hair back into a ponytail.

Following the six-month timeskip, Ishimaru appears with significantly shorter hair cut just above his ears, along with side bangs replacing his earlier middle part.

Ishimaru is portrayed as calm, eccentric, and seemingly apathetic, often staying up all night reading manga and books. His chronic sleep deprivation contributes to the dark circles beneath his eyes. While he can be sly occasionally eavesdropping on the F Squad’s plans to form his own strategies he does so only when he deems it necessary. He finds formalities tedious and prefers casual speech, often addressing others with unique nicknames such as “Miss Hostage” for Tsukasa Taira and “Mr. Foreigner” for Johannes Kleeman. His honesty and blunt manner can lead him to make statements that are occasionally misunderstood or interpreted as inappropriate.

Despite his detached demeanor, Ishimaru demonstrates strong leadership skills, encouraging the F Squad in his own way and aiming for collective success in their missions. He has a deep fascination with devils, particularly their physical abilities, which he believes should be used to their fullest potential. This fixation, however, initially left him insensitive to the emotional struggles of devils, as shown when he forced Yuuki Anzai to transform without considering the psychological toll. While CCC leader Kirio Kikuhara described him as “a quiet man” with “a deep understanding of devils,” Takashi Sawazaki criticized him for lacking essential empathy.

Highly intelligent and calculating, Ishimaru can fabricate convincing false statements and work histories without detection. His poker face and level-headedness make it difficult for others to read his intentions. During his undercover work within the CCC, he successfully concealed his true identity and motives from the F Squad. Although his infiltration was purely to gather intelligence, he earned the trust of CCC members while remaining skeptical of the F Squad’s investigative competence, as they failed to uncover his deception until much later.

Following his departure from the CCC and the disbandment of the F Squad, Ishimaru began to show more of his genuine personality. He developed a greater understanding of devils’ emotions, grew to appreciate the F Squad’s camaraderie, and became more considerate toward others. He openly thanked Tsukasa for saving Yuuki’s life referring to Yuuki as “one of my men” and was the only one to comfort her during her emotional breakdown after her temporary separation from Yuuki. His ability to read between the lines allows him to anticipate events and navigate delicate situations effectively.

Ishimaru describes himself as someone who needs a pretext for his actions, such as when he visited former colleague Yousuke Asami. While he claimed there was a specific reason for the visit, his true motive was simply to check on Asami and ensure his well-being.

The given name Megumi (恵巳) combines megu (恵), meaning “favor” or “benefit,” and mi (巳), meaning “snake.” His surname Ishimaru (石丸) consists of ishi (石), meaning “stone” or “volume measure,” and maru (丸), meaning “round,” “circle,” “pills,” or “purity.”