The biggest bad of them all is Lucifer. Ahriman, in Zoroastrianism. Called other things in other religions. He is the baddest of the bad, the head honcho, the Devil himself.
But what if you want to use him in a novel?
It's more or less a given that the big bad is BAD. Can he be reformed? I encourage you to try, but while his underlings can be, I personally wouldn't mess with his malicious nature. I don't think I've ever read a romance where Lucifer was the hero. The bad guy? Certainly. Maybe one of his underlings or even his son or daughter was reformed. But not him.
Why is he untouchable? Because there always needs to be a big bad in Hell. If not, the whole order of nature disintegrates. How are you supposed to measure evil when the stick you normally measure it against has suddenly become softhearted? While I have seen him front and centre in some books -- in Christine Warren's short story, Devil's Bargain, for instance -- his starring role has been limited to antagonist or overlord, never good guy.
Now that I've come to the conclusion of my Angels and Demons miniseries, I'm going to talk to you about my own writing! Check back in the next couple weeks to read three free reads and choose which series you'd most enjoy reading!