This is just an idea I may or may not expand on in the future, feel free it take inspiration
He wasn't always like this; he wasn't always a just a ball of negativity. He used to be something else, someone else. He knows this; he has, he first came to, been Misery, both in name and spirit. He was a curse, a spiritual being not many could see. Misery had been incarnated several times, yet he was always a form of itself – distress, suffering, anxiety, and so much more. Misery was not human, never human, but that doesn't mean he didn't know what it means to be human.
He was close to being human once, yet he was more cat than human, more demonic than they should ever be, and the humans? They hated him for it. He was a curse; he was Misery incarnate, and he was strong. Misery was something many felt; it wasn't a surprise that he would've been the one to gain consciousness, to truly remember who he was, and when he did? Well, he remembered someone he loved—quite a few, actually—and he was determined to find them again.
He searched and searched and searched, and he found them: his sister, his brother. They were his everything.
She remembered vague flashes of being someone, something else. She didn't understand them, not at first. Then she evolved; she felt the energy she was made of, wild and so very something, lash out. She was madness, the insanity of men. She was all that they feared they would be. Madness was quite familiar with the fear of humans; she was familiar with the ways she was treated, chased, and almost killed. She understood her existence was like a stain in human history; she was the natural enemy of humans, the one thing that would always want to kill them, the one thing they'd always want to kill.
Humans weren't sympathetic towards beings like Madness, and she wasn't sympathetic towards humans. But then, then she found a little kid, one who felt achingly familiar, and she couldn't help but think, "Oh..." because that was her once. Once upon a time, she'd been human; once upon a time, she had lived, grown, eaten alongside humans. She had been accepted around them, loved even, and she craved that love again; she wanted to be with them again.
Then he found her, and together they found him.
He wasn't fully human, but he was never mistaken for a curse either. He was an Angel, or that's what everyone believed. He was a being created of both humanity and positivity. He was a prodigy in all healing, in making talismans and ice manipulation. He was respected and looked at with awe, a servant, nothing more than a protector (an unneeded one at that) to his God. A selfish and arrogant little kid with bright hair and brighter eyes, an heir, and a strong one.
He was an Angel, a servant of God in this world, in this cruel and horrible world, but not all angels had to follow. There was one. He had betrayed his God and was punished by it, never to enter heaven again. Maybe, just maybe, he wanted that—to be free of the shackles his Clan had put him in. He never asked to be an Angel. He never asked for his wings to be chained. Then they found him, and he remembered, and he couldn't be happier to take on the role of Lucifer, to be the betrayer, though he won't be starting a rebellion.
He was (finally) happy.