Lucifer Morningstar/History

Past
Prior to the creation of the Earth, Lucifer was an angel, said to be a dreamer with fantastical ideas for all creation, but the elders of Heaven saw him as a troublemaker who posed a threat to the order of their world. He watched as the angels began to expand the universe and the creation of Adam and Lilith. After Lilith left Adam, Lucifer met her in the Garden of Eden and the two dreamers fell in love with each other. Lucifer, with Lilith, wanting to share the power of free will, offered Adam's second wife, Eve, the Fruit of Knowledge, with this act evil made its way into the Earth which subsequently created Hell. As punishment, both Lucifer and Lilith were sent to Hell, where Lucifer could only see the cruel and the wicked that came from humanity. Ashamed, Lucifer lost his will to dream.

In Hell, Lucifer, now a fallen angel, became the ruler of all Hell on the basis of his angelic power, ruling over all the Demons that reside there. At an unknown point, Lucifer became part of an order of powerful demons called the Seven Deadly Sins, each member embodying a sin, with his being Pride. At another point, he married Lilith, and later fathered a daughter named Charlie Morningstar with his wife.

Roughly seven years before the events of the main series, Lilith left Lucifer for unknown reasons. This sent Lucifer into a deep depression over the loss, and isolated himself from his kingdom and daughter as a means to cope.

In present day, the reclusive Lucifer spends his time in his home coping by crafting rubber duckies. He is mostly inactive ruling over Hell, leading to many other demons, such as the Overlords, Seven Deadly Sins, and Ars Goetia Demons, to do the actual ruling of Hell in his place. Due to his absent presence, many of the sinners don't respect his or his family's authority despite being the royal family of Hell, albeit, they are at least aware of Lucifer's immense power, leading them to leave his daughter alone despite how much she annoys them.

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 Sins, Mammon, created a rip-off of his theme park called Loo Loo Land, claiming to be a friend of Lucifer.

Hazbin Hotel pilot
In the pilot's opening song, shadowy figures resembling him appear.

During Charlie's fireworks display after the Extermination, Carmilla Carmine, Zestial and Zeezi are seen watching from a window along with a red-eyed silhouetted figure of Lucifer.

He is later mentioned in Charlie's voicemail to Lilith.

Several photos of him can be seen throughout the Happy Hotel, most notably a family portrait that catches Alastor's attention.

"Overture"
Charlie receives a phone call from her father informing her that the leader of the Angel Army wants to meet and asked if she could go to the meeting in his stead.