The Gundam Barbatos Lupus fought a fierce battle against the Hashmal in the vicinity of Chryse, a city on Mars and was so severely damaged that it was sent to the Saisei's MS workshop for a major upgrade.Data sampling of Barbatos Lupus’ fight with Hashmal was also conducted during the overhaul. To match the combat data of Mikazuki Augus, who was increasing his compatibility with the mobile suit through the Alaya-Vijnana System, the machine's response speed was increased to the limit of its theoretical value. Movement of the frame's joints were also delicately tuned to match the pilot's actual sensations. The suit's cockpit hatch was changed from its predecessors' top opening method to a front opening style in consideration of Mikazuki's worsened disabilities.

To more intuitively reflect Mikazuki's fighting style, the suit's arms were enlarged and elongated like a beast, and various weapons are fitted on the body. The arms' frame structure was also upgraded and reinforced. The tail parts of the Hashmal was transplanted to the suit's back, and through the Alaya-Vijnana System, Mikazuki controls this tail blade as if it was a limb.bOverall, the Gundam Barbatos Lupus Rex is customized for close combat, and its silhouette gives it a more ferocious look than its predecessors. The word 'Rex' means 'King' in latin, and this is fateful as Mikazuki is going along with Orga Itsuka, whose aspiration is to be "King of Mars."