Man, the fandom chatter around 'Hazbin Hotel' is wonderfully wild, and when people ask about Adam I usually start by saying: there isn’t an official, fleshed-out origin for an Adam in the pilot or the creator’s major releases. What most of us call “Adam” is a fan-created figure or a heavily extrapolated minor presence, so his backstory lives mostly in fanfics, art, and headcanons rather than in canonical source material. That freedom is honestly part of the fun—people lean into different eras and sins to build him.
In the most common fanon version I’ve seen, Adam is a human who burned bright with ambition—maybe a slick executive, a relentless politician, or a charismatic performer—whose choices and hubris led to a dramatic death. In Hell he becomes a demon shaped by that original sin (pride or ambition), gaining a veneer of charm with a razor-sharp edge underneath. Depending on the author, he’s written as a rival to charismatic figures like Alastor, a tragic romantic interest, or a manipulative fixer who knows the bureaucratic angles of Hell. If you want to dive deeper, check fanfiction sites and the 'Hazbin Hotel' wiki, but go in knowing a lot of what you’ll read is creative reinterpretation rather than straight canon.