Fastener presents as a pink plushie-like entity with angular features and striking light green eyes. Two prominent zippers define his form: one vertical along his forehead, the other encircling his neck like a collar. His limbs are short, black, and pointed, functioning as sharp scythes that dismember foes.

He serves as a familiar spirit to demonic sisters Scanty and Kneesocks, diligently performing duties like driving their vehicle G-String and maintaining their residence. Despite unwavering reliability, he suffers frequent physical abuse and heals injuries slower than his counterpart Chuck. Communication occurs solely through vocalizing his own name.

Fastener can unzip his body to unleash a spectral form resembling a two-headed wyvern with vertically split necks. This manifestation attacks using formidable claws, though its regeneration remains vulnerable; adversaries like Chuck exploit this by rapidly healing and countering with energy blasts. His scythe-limbs also deploy traps, typically targeting Chuck rather than the Anarchy sisters.

Personality reveals heightened focus and intelligence compared to Chuck, driving Fastener’s undivided dedication to tasks. This competence fuels their rivalry, originating at Zipper University where Fastener’s popularity clashed with Chuck’s obscurity. Their feud escalated when both pursued the same cheerleader; Fastener won her affection via a superior gift, culminating in a sexual encounter.

Unzipping Fastener’s cranial zipper accesses a tiny female devil within his head. This entity nurses romantic feelings for an analogous devil inhabiting Chuck’s head, complicating their enmity.

During the second season, Fastener persists as Scanty and Kneesocks’ loyal servant following their exile from Hell. He adapts to their new alliance with the Anarchy sisters, aiding their collective pursuit of Hell Coins for redemption.