The Obsidian Goddess, known in Japanese as Kokuyō no Megami, is a mysterious supernatural entity that resides within the body of the protagonist Musashi. She is depicted as a female figure with long fair hair, sometimes shown with a halo, and she can freely change her shape between the form of an adult woman and that of a teenager. Within the story she is presented as the awakened form of an enigmatic presence that sleeps inside Musashi, and she is the target of an organization known as the Obsidian Eight, who pursue her in order to obtain her power, particularly the strength they believe she can provide through the legendary Kitetsu Blades.

Her background is tied to the oldest conflicts of the world she inhabits. According to the lore revealed in her backstory, she was originally one being with the first Black Demon God, but she betrayed and separated from that entity so that she could find an ally capable of stopping him. She has a long connection to the scabbard clan, the lineage to which Musashi's friend Kojiro belongs, and she is said to have acted as a kind of compass that guides people toward legendary weapons and treasure. Secrets and relics concerning her are preserved in the mausoleum of the Uesugi clan, and her origins become a subject of investigation for the characters as the story progresses.

In terms of personality, the Obsidian Goddess is blunt, critical, and demanding. She openly confronts Musashi about his inner weaknesses, telling him that he is willing to do anything to feel wanted and that his desire to become a Bushi is only an excuse to remain close to his friend. She judges that a person with such uncertain motives cannot truly wield a Kitetsu Blade, and she is noticeably harsher toward Musashi than she is toward Kojiro, whom she treats more gently because of his connection to the scabbard clan. She prefers to remain hidden and is selective about when and to whom she reveals herself.

Her primary motivation is to stay concealed and to eventually pass her power on to another member of the scabbard clan. This desire explains her protective fondness for Kojiro and her reluctance to fully embrace Musashi as a vessel. She does not seek to dominate or control events openly; instead, she remains in the middle of conflicts while keeping her own intentions private.

Her role in the story is central, as the Obsidian Eight actively hunt her and the factions around her treat her power as a decisive advantage in their battles. At a critical moment, she chooses to grant Musashi her power and demonstrates how she fights by using his body. During that demonstration she draws in blade energy from every Kitetsu Blade around her and seizes an opponent's weapon, overwhelming her enemy and saving those who were threatened. Her abilities are also sought by military leaders who believe that unlocking her power could provide the edge their forces need against powerful demons.

Her notable abilities revolve around control over blade energy, or tōki. She is described as a ruler of blade energy who can draw it in and manipulate it freely, making her exceptionally dangerous in any conflict involving Kitetsu Blades. Combined with her shapeshifting and her apparent ability to guide others toward legendary weapons, she is a figure of immense strategic and combat value.

Her development in the story follows her gradual shift from rejecting Musashi to cooperating with him. She begins by dismissing him as unworthy and incapable, but eventually decides to bestow her power on him and fight alongside him through his body. Their relationship becomes a central dynamic in the narrative, with the goddess slowly accepting the role her host must play even as she remains focused on her deeper goal of passing on her power to the lineage she truly favors.