Dex is one of twin Mecha-Ude life forms symbiotically bonded to Aki Murasame. These sentient mechanical beings originate from another dimension, compressing into arm-like forms to survive on Earth by fusing with human hosts and accessing vital arbitrium energy. Dex and his twin Sinis manifest as oversized mechanical fists emerging from seal marks on Aki's thighs, featuring black and grey coloring, vermilion neon lines, and visible eyes. Their design became more angular and less bulky in the full series compared to earlier versions.

Dex barrels through situations with a crude, impulsive, and cheerful personality, forming a direct counterpoint to Sinis's composed nature. This contrast establishes them as polar opposites. Energetic and quick to advocate violence, Dex often serves alongside the more measured Sinis as a voice of reason for Aki. Both address her as "Aki-sama" and function as loyal partners rather than mere tools, though Dex's demeanor remains overtly aggressive. Sustaining their partnership demands significant arbitrium, requiring Aki to consume large amounts of food.

In combat, Dex and Sinis utilize rocket-propelled elbows for high-mobility maneuvers and projectile attacks. They act as extensions of Aki's fierce fighting style, their menacing appearance and coordinated actions amplifying her presence. Dex originated as the Mecha-Ude bonded to Aki's twin sister, Fubuki Murasame, before transferring to Aki, a history potentially contributing to his contrasting personality with Sinis. Forced separation from Aki would trigger fatal arbitrium leakage for her while causing Dex to regress into an inert cube form.

Throughout their appearances, Dex and Sinis remain steadfast protectors, safeguarding Aki in conflicts against adversaries like the Kagami Group and later against Fist's machinations. No significant individual character evolution or standalone background for Dex beyond this partnership is detailed.