Jirō Yamada, also known as MC.M.B (Middle Brother), serves as a core member of the Ikebukuro rap group Buster Bros!!! alongside his older brother Ichiro and younger brother Saburo. Born February 6th, this 17-year-old high school student handles physical labor duties for the family business, Odd-Jobs Yamada. Standing roughly 180 cm tall, he possesses striking complete heterochromia: his left eye is forest green, his right eye gold. He bears a mole beneath his left eye and another below the left corner of his mouth. His signature look features a black t-shirt with dual yellow zippers, a sapphire blue-and-white bomber jacket adorned with Buster Bros!!! patches, ripped grey jeans, black platform sneakers boasting blue laces, and multiple rings. He frequently adds a red-and-navy snapback hat and a plaid shirt tied around his waist, decorated with "B" and "M" badge pins.

Sociable, athletic, and popular with peers, Jirō exhibits a fiercely protective hot-headedness, especially when his brothers face insult or threat. His personality initially centered on idolizing Ichiro, mirroring his brother's delinquent persona and interests in anime and light novels. This deep admiration originated from a pivotal childhood event: after their parents abandoned them, Ichiro rescued Jirō and Saburo from kidnappers linked to the Stairway to Heaven gang (led by Mozuku Shito). This act restored Jirō's shattered trust and respect for Ichiro. Over time, however, Jirō recognized his excessive reliance on Ichiro's identity, prompting him to seek his own individuality while preserving brotherly respect.

Jirō's relationships are deeply rooted in familial loyalty and conflict. He now addresses Ichiro as "Aniki," a shift from "Nii-chan" marking his move from blind worship towards desiring equality. With Saburo, he engages in intense rivalry for Ichiro's approval, leading to frequent verbal and physical clashes. Despite this friction, Jirō demonstrates fierce protectiveness, sacrificing himself to shield Saburo from danger. Their dynamic improves as Jirō establishes his own path, though antagonism endures. He harbors profound animosity towards figures like Jyuto Iruma (who mocked Saburo's anxiety) and Rio Mason Busujima (who infantilized them). He also maintains an informational rapport with Urumi Asogi, a Shinjuku underworld informant who teases him yet genuinely cares for his well-being.

A pivotal development unfolds when Rei Amayado, their biological father, reveals himself, contradicting Ichiro's longstanding assertion that their father was dead. Jirō vehemently rejects Rei, defending Ichiro's deception and showcasing unwavering loyalty to his brother. This confrontation cements Jirō's prioritization of his chosen family over biological ties. During the Yokohama container yard incident, he initially attacks Doppo Kannonzaka, mistakenly believing Doppo aided in a kidnapping, but later collaborates with him against the Tobari brothers to stall for allies.

Jirō's rap ability, "Rally," channels his teammates' strength to unleash potent attacks. His Hypnosis Mic manifests as a blue microphone with "MB"-engraved rings, while his speakers form a rock ensemble featuring Flying V guitars and amplifiers branded with the Buster Bros!!! logo.

Notable trivia includes his signature quote ("We don’t stop playing because we grow old; we grow old because we stop playing"), his increased school popularity following the Division Rap Battles, and his preference for outdoor activities like soccer, skateboarding, and running. While occasionally depicted playing guitar, he states this occurs infrequently.