Fastener appears as a pink plushie-like entity with light green eyes and a sharply angular head. Multiple zippers decorate his form, featuring one vertically bisecting his forehead and another circling his neck like a collar. His short, pointed black limbs double as scythes in combat. Inside his head resides a tiny female devil clad in a black suit, who harbors romantic feelings for the entity within Chuck's head.

Functioning as a familiar spirit, he loyally attends demon sisters Scanty and Kneesocks. His duties range from driving their vehicle G-String to maintaining their residence. Though he communicates exclusively by repeating his name, he demonstrates superior focus and intelligence in tasks compared to Chuck. Fastener routinely suffers physical abuse and injuries, healing regeneratively but slower than his counterpart.

His rivalry with Chuck originated at Zipper University, where Fastener enjoyed popularity while Chuck endured isolation. Their conflict escalated when both pursued the same cheerleader; Fastener won her favor with a superior gift, resulting in a sexual encounter that intensified their hatred. This antagonism continues in their current roles, with Fastener dismembering Chuck during clashes and laying traps designed specifically for him.

In battle, Fastener's limbs morph into sharp scythes for offense. By unzipping and inverting himself, he accesses a formidable ghost-wyvern form—a two-headed (or vertically bisected) dragon with wings, attacking via massive lethal claws. Despite this form's power, Chuck once overpowered him in a transformed state.

Immobilization occurs when his head zipper opens, releasing the diminutive female devil inside. She maintains a romantic relationship with the incubus inhabiting Chuck's head.