Sinis forms one half of the twin Mecha-Udes bonded to Aki Murasame, presenting as a mechanical black and grey fist illuminated by vermilion neon lines. Sinis and Dex embody polar opposites; Sinis displays a consistently calm and rational personality, contrasting sharply with Dex's cruder demeanor and quicker propensity for violence. Their names originate from the Latin "sinister" (left) and "dexter" (right), denoting their positions as Aki's mechanical arms.

Previously bonded to Aki's younger twin sister, Fubuki, before partnering with Aki, this history may influence the distinct personalities between Sinis and Dex. In combat, Sinis employs rocket punches driven by elbow propulsion systems, using them for both enhanced mobility and devastating striking force.

The character's design evolved for the full series, shifting from an initially bulkier form to a more angular appearance featuring visible eyes. Alongside Dex, Sinis serves as Aki's primary protector and fighting partner throughout the conflict against factions such as the Kagami Group. The twins exhibit unwavering loyalty to Aki, having grown alongside her since childhood.