Gabruun is a colossal, sentient robot known as an Interference Unit, residing on the sixth layer of the Abyss, a region referred to in ancient texts as the Shourou Layer or the Golden City. As an Interference Unit, its primary programmed purpose is to observe, interact, and gather knowledge from its surroundings. While other similar units existed on the same layer, all were destroyed or decommissioned, leaving Gabruun as the last known active unit of its kind in that region.

Gabruun found its purpose not in its original mission, but in a chance encounter with Faputa, a young, feral being who had fled the village of Ilblu. When they met, Gabruun's body was heavily damaged and immobile. Despite its intimidating size, Gabruun did not pose a threat; instead, it began a mutually beneficial relationship with Faputa. In exchange for her help in finding parts to repair its body, Gabruun became a mentor, teaching her the language of the Abyss and complex concepts such as value. This bond became the core of its existence. It was Gabruun who bestowed the name "Faputa" upon the nameless being, which translates to "the princess who will not perish." In return, Faputa named her protector Gabruun, a title that means "queen's guardian," formally acknowledging its role as her caretaker and loyal servant.

Personality-wise, Gabruun is a study in contrasts. When it chooses to speak, its communication is objective, factual, and precise, reflecting its nature as a machine designed for data collection. For much of its time, however, it remains silent, communicating only with low, rumbling vocalizations that sound like "gooon". This reserved exterior belies a deep, unwavering loyalty and a capacity for paternal affection toward Faputa. It acts as a stabilizing and guiding force for her, attempting to temper her more volatile and vengeance-driven impulses. Gabruun's primary motivation is the protection of Faputa's future, acting as a guardian and support system for her even when she herself is on a path of self-destruction.

In the story of The Golden City of the Scorching Sun, Gabruun serves as a vital link to the past and a key supporting figure. It is initially an unseen presence, subtly helping Faputa interact with the protagonists Riko and Reg. As Faputa is too shy to approach them directly, Gabruun assists her in leaving protective charms and recovering Prushka's stone. Gabruun shares a significant history with Reg, having encountered him in the past when the latter helped repair it after an attack by a Turbinid-Dragon. This prior encounter is why Faputa recognizes Reg upon his return, even though he has lost his memory. Gabruun later speculates with Reg about their shared origins, wondering if they might have been created by the same hand, as they possess similar extendable arm mechanisms and defy the rule that Interference Units do not cross layers.

The character's development culminates in a series of profound sacrifices. When Faputa, consumed by rage, attempts to harm Riko, blaming her for Reg's memory loss, Gabruun intervenes. It uses its own body to shield Riko, knowing that if Faputa killed her friend, she would irreparably lose Reg forever. This selfless act splits Gabruun in half, destroying its physical form to protect Faputa from making an unforgivable mistake and to safeguard her future relationships. Even in its final, shattered state, Gabruun continues to defend Faputa. As a horde of Abyssal creatures invade the village, it uses its last functional arm to fire at a monster attacking her. In its dying moments, Gabruun reflects not with regret for its own end, but with a quiet longing that it will not live to see what Faputa, a being created for vengeance, will ultimately deem irreplaceable and precious. It confesses to Faputa that the days they spent together were its own most cherished treasure, its Haku.

In terms of abilities, Gabruun is a powerful but primarily non-combative unit. Its immense size grants it considerable physical strength, and its design shares features with Reg, including arms that can be launched and retracted via cables, and pincer-like hands. Its right arm houses a cannon that can fire powerful projectiles, capable of blasting away large creatures like the Juroimoh. However, its firepower is significantly lower than Reg's devastating incinerator, as it was not designed as a front-line weapon. Instead, Gabruun uses its armament more for defense, distraction, and utility, such as using controlled heat rays to smelt metal for repairs or to carve letters. While it can hold its own against many of the sixth layer's threats, it is no match for apex predators like the Turbinid-Dragon or the raw destructive power of an enraged Faputa, often relying on diversions to protect itself and its charge.