Gonne

"Lucifer" does show up in the Bible, in Isaiah 14:12. Though, as you said, I'm pretty sure he's being used here as a metaphor for a prideful king. Some translations just say "Morning Star" like the NIV, and for the French translations it's often "Astre brillant, fils de l'Aurore" which is basically "Brightful star, son of the Dawn".

In fact, I wouldn't be surprised Lucifer most of the time is used as a metaphor in the Bible, rather than a "big bad demon." For example, there is also mention of a prideful angel leader in the second book of Enoch (in which he's named Satanael) and in the book of Revelations, where there's a mention of Satan fall after being cast out by Michael and his angels (12:9).

In the books of Enoch, the fall of Satanael could have been a prefiguration of the fall of mankind from Eden. At least, that's an argument I saw. It could have foretold a sort of "chaos" from inside Heaven, which eventually also started to explore the concept of free will given to each being.

In Revelations, it's important to remember the political context in which it was written, where early Christians were persecuted and killed under the Roman occupation. In fact, from lessons I remember and books i have read, Revelations could have been written to give hope to believers, promising to them an eventual justice and peace, while promising death and condemnation to their oppressors. A theme often retrieved in apocalyptic writings. In such a context, the weird and sometimes terrifying visions seen by the author are meant to be put in relation to the context when it was written, as they could actually be hidden metaphors.

Also, a fun fact: in some texts, such as in saint Jerome's Vulgate, Lucifer was actually one of the names given to Jesus, as it meant "bearer of light." It can be observed as an example in the Latin translation of the second letter of Peter. Jesus is also linked to the Morning Star in Revelations.

In the end, if I had to summarize everything quickly: I think you're right it is about a human king in the Bible and maybe not THE devil, as it would not be the first case Lucifer or Satan is used as a metaphor in his fall.

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Now, if we speak about my bible lore!

Lucifer does exist as a fallen angel! He was actually one of God's angels, and he was a favorite as Heylel. However, things went bad when God decided to offer free will and omniscience to him, as Lucifer couldn't handle such intensity. He interpreted it in a VERY different way God planned it originally lmao.

Eventually, that led to him plotting to overthrow his father, which failed and got him to fall in a new dimension away from Him, which Lucifer moulded into Hell.

In the normal bibleverse I'm doing, Lucifer will maybe appear to Jesus and Judas in the main storyline. He tends to taunt and be aggressive to Jesus, because he refused to submit and work under his orders in the desert. It's very possible Lucifer will try impersonating Jesus to mess with Judas' mind, as he is quite skilled with impersonation.

Lucifer is quite dangerous as he tends to get spiteful and offensive when put in Jesus' presence, when most of the other demons flee and cower at Jesus' sight. Lucifer is also a shapeshifter, and there's already been scenarios of him taking Jesus or the other apostles form to scare some characters around. Eventually, one of the ways to find out if it's actually him is to look for weird marks on the wrist, ankles, and flank.

I'm gonna be honest, I do not think that Lilith is canonically or officially the first wife of Adam. Why? Because Lilith' story of being the first wife of Adam originated from the Alphabet of Sirach, a Hebrew text from the middle age, and some sources say it could be a satirical text.

Lilith also seems to be, as it is in many jewish/christian demons backstory, inspired by pagan divinities and spirits. So I would argue, at least in my opinion, that this is another case of an old deity being turned into a monster. In this instance for Lilith, an old deity said to cause infertility and maternity issues turned into a succubus and a baby-killer.

Like you said, the only mention of Lilith in the Bible would be in Isaiah 34:14, and it seems to support the argument of Lilith being a "monster" as some translation used "night-monster" (ERV) or "screech owl" (KJV). There's no mention of "the lilith" being the fallen first wife of Adam.

However, I do agree that with the later retelling, an importance was given to Lilith being Adam's first wife, and she's nowadays a sort of feminist figure for having refused to submit to a man's will (at the cost of being casted out and cursed). Lilith seems to be notably rebranded sometimes as Jewish feminist character, and when searching around her tag on Tumblr, she can be a deity worshipped in witchcraft/theist/pagan circle (please do note I'm NOT an expert on this, though.)

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Now, in my lore, Lilith is quite a strange figure. From Lucifer's words, she appeared after their fall, but nobody seemed to have ever really noticed her presence apart from him.