Gau Meguro is a sixteen-year-old boy who comes from the ordinary, non-ninja world and becomes a member of the Grey Wolves, also known as the Kairoshu, serving as part of the Wakachi unit alongside Raikou Shimizu. His entry into the hidden world of ninja known as Nabari was not a choice but a direct result of a traumatic event from three years prior to the story's start. After his mother was murdered and her killer was released from prison with a sentence that seemed too lenient, Gau sought revenge but was overpowered and nearly killed. Raikou appeared and killed the assailant, saving Gau's life. In the aftermath, Raikou brought Gau into his world, and Gau, feeling immense gratitude and devotion, chose to dedicate his life to serving him.

Physically, Gau is described as lean and of unimpressive stature, with fluffy black hair and brown eyes that have a cat-like quality. He tends to dress neatly, often wearing school uniforms, sweater vests, and dress shirts with ties, even while on duty. As a member of the Wakachi, he wears a yin-yang bracelet on his right hand, and later in the story, he and Raikou wear matching outfits, including butterfly pins and coordinated cardigans, symbolizing their deep bond.

Gau's personality is defined by a potent combination of fierce loyalty, explosive temper, and strong idealism. He is easily incited and quick to anger, a trait that earned him the nickname Tenpa-kun from his colleague Yukimi, which comes from the slang tenparu meaning about to blow a fuse. His temper often flares when he perceives a lack of respect toward Raikou, when faced with disorder or uncleanliness, or when his strong sense of justice is offended. He frequently quarrels with Yukimi over the state of his dirty apartment, and he is known to call out even superiors for what he sees as dereliction of duty. Despite this volatile exterior, Gau is otherwise often quiet, polite, and reserved, preferring to stay in the background, handling paperwork and other supportive tasks rather than seeking the spotlight. He is deeply compassionate and cares for those in difficulty, sometimes impulsively putting himself in harm's way to protect others or to try and resolve a situation without violence.

Gau believes wholeheartedly in the mission of the Wakachi, which he sees as protecting the ordinary surface world from the evils of Nabari. He is an idealist who believes that the ultimate secret technique, the Shinra Banshou, could be used to save the world. His primary motivation, however, is his devotion to Raikou, who is the center of his universe. Gau is extremely devoted to him, not just out of gratitude for being saved, but out of a genuine desire to see him happy. This loyalty is so profound that he is willing to betray Raikou's wishes if he believes it is for Raikou's own good. Knowing the true, tragic motives behind Raikou's massacre of his own clan, the Shimizu clan, Gau seeks out Raikou's sister, Raimei, and reveals the truth to her, offering his own life as proof, hoping to heal the rift between the siblings.

This act of devotion leads to a confrontation where Gau throws himself between Raikou and Raimei, taking a fatal blow from Raikou's sword that was meant for his sister. This injury leaves Gau in a coma. Raikou, though grief-stricken and blaming himself, refuses to use a forbidden technique from the Koga village that would require human sacrifice to wake Gau, knowing Gau would never want anyone to die for him. Gau is eventually awakened by the kira technique of Yoite, a dying boy seeking to erase his own existence. Feeling deeply indebted to Yoite for saving his life, Gau makes the momentous decision to go against the Grey Wolves and pledge his support to Yoite, aiding him in his escape and helping him despite the danger. This marks a significant development in his character, as he shifts his unwavering loyalty from Raikou to also encompass Yoite, demonstrating his capacity to act on his own moral compass. In the anime adaptation, after Raikou's death, Gau moves to Banten and joins the kendo club as a tribute to him. In the manga's epilogue, he survives the final battles and moves to Banten together with Raikou. The series creator has suggested that the exact nature of his relationship with Raikou is left open to reader interpretation.

Gau is not a powerful shinobi and admits to having no prior connection to the Nabari world. He cannot handle a sword properly and has poor stamina for fighting. His value lies not in combat but in his intelligence, meticulous nature, and practical skills. He excels at research, compiling reports, and handling administrative duties. His notable abilities include reading Braille and using a three-part abacus. He is also a skilled cook, particularly enjoys baking, and is fastidious about cleaning and organizing, always seeking to be useful in non-violent ways. He is a percussionist who plays the timpani in his school band and likes Pacific saury, honey, and classical music while disliking shiitake mushrooms and messy rooms.