Lucifer Morningstar/History

Past
While his origins are widely obscured, Lucifer is said to be a being known as a Fallen Angel and is the unquestioned ruler of all Hell on the basis of his angelic power.

At an unknown point he married Lilith and later fathered a daughter named Charlie Morningstar with his wife.

At some point, he started a successful theme park called Lu Lu World that proved to be quite popular with the denizens of Hell. Another of the Seven Deadly Sins, Mammon, created a rip-off of his theme park called Loo Loo Land, and claims to be an acquaintance to Lucifer.

Hazbin Hotel pilot
Though he plays no role in the pilot, Lucifer is referenced and alluded to many times. Shadowy figures resembling him appear during the opening song "I'm Always Chasing Rainbows". Notably, during the verse "Why have I always been a failure", his shadow materializes behind Charlie, and is later mentioned in Charlie's voicemail to Lilith. Both possibly suggest that he is disappointed in her and her decision of starting up the Happy Hotel.

Later during the fireworks display, four demons are seen watching from a window, one of which is a red-eyed silhouetted figure wearing a large hat and holding a distinctive cane, features that strongly suggest it is Lucifer.

His visage is later glimpsed at in several background images of the Happy Hotel, most notably a family portrait that catches Alastor's attention.