Melan Blue is a central figure in the narrative, serving as the primary protector of the young girl Marin Asagi. He is a Monomakia, a type of artificial living weapon, specifically classified as a Gun-Swordsman, originating from the otherworldly realm known as Brigadoon. His existence begins on Earth when Marin discovers him sealed inside an ampoule at a local shrine, and his release marks the start of their intertwined journey.

In terms of physical appearance, Melan Blue is tall and humanoid, clad in distinctive blue armor. His form is explicitly designed for combat. He possesses a set of mechanical wings that extend from his back, granting him the ability to fly. In place of hands, his right arm is mounted with a narrow, single-edged sword, while his left arm features a powerful laser rifle. He also has a set of retractable tentacles that emerge from his chest, which he uses to grasp objects or hold onto Marin during flight. A unique and somewhat unusual requirement for his functioning is his need for rice, which serves as his fuel; he is capable of consuming up to fifty bowls of rice in a single sitting.

Personality-wise, Melan Blue is initially depicted as a quiet, reserved, and almost unemotional being. He speaks in a very direct, straightforward manner, often appearing to lack a full grasp of social cues, idioms, and human customs. This behavior is not born from malice but from a complete lack of experience, as his background involved intense and harsh training that prioritized combat efficiency over interpersonal skills. He is incredibly focused and single-minded, dedicating himself entirely to his mission of protecting Marin. Despite his stoic and sometimes robotic exterior, he possesses a strong sense of honor and justice, showing courage beyond measure in battle and refusing to stop fighting even when critically injured. Over time, his interactions with Marin and her family begin to draw out a more protective and even gentle side, revealing a deep-seated loyalty and a budding capacity for emotion.

Melan's primary motivation is his sworn duty to protect Marin. This mission stems from events a century before the main story, when a figure from Brigadoon entrusted him with the Creis Marine, a vital entity necessary for a ritual that separates the worlds. He stole the Creis, sent it to Earth, and then sealed himself away to await its awakening. Upon being released by Marin, whose biological data closely matches that of the Creis, he believes his purpose is to safeguard her from the renegade Monomakia that are sent to eliminate her. His role in the story is that of a bodyguard and warrior, constantly defending Marin from relentless attacks while simultaneously struggling to understand the strange, peaceful world he now inhabits.

His key relationships are central to his development. The most significant is his bond with Marin Asagi. Initially, he views her solely as a charge to be protected, but her persistent kindness, emotional honesty, and her insistence on calling him a friend gradually erode his emotional barriers. He learns about human feelings, such as sadness, humor, and eventually love, through his experiences with her. He also forms connections with Marin's adoptive family, the Asagis, who accept his strange presence without question, providing him with a taste of domestic life and unconditional acceptance that further aids his journey toward understanding humanity.

Throughout the story, Melan Blue undergoes significant development. He transforms from an emotionless, duty-bound weapon into a being capable of deep feeling and personal desire. He starts to question his singular focus on his mission and begins to act out of a genuine wish to stay with Marin and her family, rather than out of pure obligation. He learns to express his thoughts and feelings more clearly, moving from brief, robotic statements to more heartfelt communication. The narrative challenges his understanding of duty versus personal bonds, forcing him to confront his own identity beyond that of a mere protector.

As a Gun-Swordsman, Melan Blue possesses formidable combat abilities. His integrated sword and laser rifle make him a threat at both close quarters and long range. His mechanical wings give him aerial superiority, allowing him to engage flying or ground-based enemies from above. His retractable tentacles provide additional utility, enabling him to manipulate his environment and shield his charge. A remarkable ability is his capacity for self-repair, as he can regenerate from significant damage, though this process has limits and can be overwhelmed by severe injuries. His calm and calculating demeanor in battle makes him an effective strategist, often remaining composed and analytical even in the most desperate situations.