Otaru Mamiya is the central protagonist of Saber Marionette J. At the start of the story he is an eighteen-year-old day laborer living alone in the city-state of Japoness, a society whose all-male population relies on feminine androids called marionettes for labor and companionship. He supports himself through modest work, selling fish and handmade goods such as parasols, and he trains in martial arts at a local academy, where he develops solid fighting skill. He is portrayed as an ordinary, good-natured young man who gets along with his neighbors and takes part in community life, but his existence changes when a series of mishaps carries him to the abandoned Japoness Pioneer Museum, where he accidentally awakens Lime, a marionette equipped with a rare maiden circuit that grants her genuine emotions. He later awakens two more such marionettes, Cherry and Bloodberry. Physically, he is a teenage boy with a distinctive two-tone hairstyle, light brown in the front and dark brown in the back.

In terms of personality, Otaru is stubborn and determined, but also gentle and frequently overwhelmed by his new circumstances. He is initially presented as largely uninterested in romance and is constantly flustered by the affectionate, competitive attention the three marionettes give him, returning their feelings in a platonic and even-handed way. Rather than treating the girls as machines or property, which is the prevailing attitude in his world, he regards them as individuals and comes to care for them deeply, eventually forming something like a family unit with them. His kindness and willingness to treat them as people set him apart from others in his society and are precisely what allow their emotional circuits to grow.

His primary motivation is protecting and nurturing Lime, Cherry, and Bloodberry. When he learns that the marionettes were designed to mature emotionally under a human tutor, he accepts the role of their guardian, teaching them through shared daily experience. As the story unfolds, he is drawn into political conflict between Japoness and the militaristic state of Galtland, and he and the girls work together to counter threats against Japoness. When Lime's maiden circuit is stolen, Otaru undertakes a desperate search to recover it, and he later takes part in attempts to infiltrate Galtland and confront its leader. Through these events, his journey carries him from a nondescript street vendor to a nationally recognized hero.

His most important relationships are with the three marionettes and with Mitsurugi Hanagata, an acquaintance whose reckless driving initially ruins his merchandise. Despite their rocky start, Hanagata becomes a recurring companion and eventual neighbor, and the two band together over the course of the story. Otaru also serves as a sharp contrast to the antagonist Faust, who treats his own emotion-capable marionettes coldly as disposable soldiers, while Otaru's warmth and respect demonstrate a different way of relating to them.

In terms of development, Otaru confronts the social prejudice that marionettes are unworthy of human affection, and he questions himself as he grows closer to the girls before ultimately accepting his bond with them. By the end of the series, his care has helped Lime, Cherry, and Bloodberry mature fully, and their bond is tested when the three choose to sacrifice themselves to free the last surviving human woman from the control of the colony ship's computer. Otaru's emotional journey is defined by his willingness to respect their choices, his grief at their loss, and the eventual restoration of their family.

As for abilities, Otaru is a capable martial artist, skilled enough to defeat Hanagata in direct combat. His signature weapon is a sai whose blade can extend into a sword and whose tail can be used to reel allies to safety. While he lacks the superhuman strength and speed of the marionettes, his bravery, loyalty, and combat training make him a dependable partner in battle.