Decim is a Threadborn of chilling flesh and colder blood, with an advanced design as if an extra efficient homunculi. Despite being very out of place within Underhaven, he "makes himself at home" working in some fashion, either with Alpine, or by himself.

In his last life, he judged souls as an Arbiter, or was some sort of cog in an unknown system of divinity. As he is now, he is meaningless, so he sticks by his de facto brother Alpine, and takes up artificery. He can hardly tell if his last life was real, or merely a product of his lonely imagination, but he chooses to believe in the black haired woman who was always by his side in his judgments.

Incarnated within the caves, he looks like a 6'2" male with fluffy white hair that glistens like the snow, and eyes like the ocean, with an Arbiter's cross to glow in the dark with him. Uncomfortably, this means he looks the same as his old life. He feels like his life is a lot more dead, without his assistant by his side, and he cannot remember what her voice or her warmth was like. He wakes up every day, praying to see her in his daily life. A home, found in a lost friend.

At present, Decim looks artificially perfect, the only asymmetry being how the shock of white sits against his silver buzz cut. He never shows emotion,and his stoicism is permanently etched into his face. All of his movements are as stiff as a mannequin's. His voice is ever calm and cordial, even if his deep tenor may aid his fit posture to make him seem more imposing and callous. He generally does not talk to anyone, and can be found accompanying Alpine in whatever the little half-elf is doing at the time.