Grospoliner, also known as the Sword Emperor or the King of Swords, is a unique and sentient weapon from the Silent Möbius universe. It is not a human character but an extra-dimensional being that takes the physical form of a massive sword. Once belonging to the powerful sorcerer Gigelf Liqueur, Grospoliner is a sapient artifact with the ability to speak, think independently, and change its shape. When not active, it compresses itself into a compact, skull-like form, but when unleashed, it reveals its true nature as an enormous blade, described as being around eight feet long with a dark brownish-black coloring, a wickedly curved edge, a horned hilt, and a long, prehensile tail that extends from the handle.

The background of Grospoliner is tied to the failed Project Gaia. Following the death of its master, Gigelf Liqueur, the sword was locked away inside an obelisk for twenty-two years. It remained sealed and dormant until it was discovered and freed by Gigelf's daughter, Katsumi Liqueur. The ritual to release Grospoliner required a contract sealed with Katsumi's blood, which allowed the weapon to bond itself permanently to her. As a result of this bond, the enormous sword becomes effectively weightless in Katsumi's hands, functioning as a natural extension of her own body.

Personality-wise, Grospoliner possesses a high degree of intelligence and wisdom, accumulated over its long existence. It serves as a mentor and advisor to Katsumi, leveraging its deep and extensive knowledge of sorcery, magic rituals, and the nature of the supernatural entities known as Lucifer Folk. Grospoliner is defined by a code of loyalty, having sworn an oath to serve the Liqueur bloodline. This manifests in its primary motivation: to guide and protect its wielder, Katsumi, and to assist her in fulfilling her destiny. It harbors a deep disdain for the entities it was often used to fight, but its primary drive is the protection of its master and honoring its oath.

In the story, Grospoliner plays a crucial role as both a powerful weapon and a source of critical information. As Katsumi's main armament, it is her primary means of combating the Lucifer Hawks that threaten Tokyo. Its physical power is immense, capable of unleashing explosive energy blasts and cutting through powerful supernatural foes. Beyond combat, its role as an advisor is equally important; it is often the one who explains the nature of the threats they face or provides historical context about the magical world, though it has been known to withhold certain information at the request of others, believing it to be in Katsumi's best interest.

The most significant relationship in Grospoliner's existence is with Katsumi Liqueur. This bond, forged in blood, is one of mutual reliance. While Grospoliner is a tool, it is also a partner and guardian, feeling a deep sense of protectiveness towards its wielder. It also maintains a communication link with other members of the Attacked Mystification Police, such as Rally Cheyenne, with whom it can converse independently of Katsumi. Its former master, Gigelf Liqueur, represents its past and the legacy it carries forward with Katsumi.

In terms of notable abilities, Grospoliner is defined by its sentience and mystical properties. It can alter its form, retracting into a dormant skull-like state and extending into its full sword form when commanded by a spell. As a weapon, it possesses immense cutting power and can serve as a focus for its wielder's magic, channeling energy into powerful ranged attacks. It also possesses a kind of spatial awareness and the ability to detect supernatural entities, functioning almost like a radar for its user. The tail that extends from its hilt frequently wraps around Katsumi's arm in combat, ensuring she cannot be disarmed, further solidifying the bond between wielder and weapon.