"The Devil You Don't" by Keith Laumer.
In the story, the Devil in the form of a shy young man comes to meet a Physics professor. He is asking for help in dealing with some kind of inter-dimensional invaders.
"Mr. Lucifer came to fix the lights," Curlene said.
"Ah—not really," the young man said quickly. "Actually, I came to, er, ask for help, Professor. Your help."
"Oh, really?" Dimpleby seated himself and stirred sugar into Curlene's cup and took a noisy sip. "Well, how can I be of service?"
"But first, before I impose on you any further, I need to be sure you understand that I really am Lucifer. I mean I don't want to get by on false pretenses." He looked at Curlene anxiously. "I would have told you I wasn't really an electrician, er, Mrs.—"
"Just call me Curl. Sure you would have."
"If you say your name's Lucifer, why should I doubt it?" Dimpleby asked with a smile.
"Well, the point is—I'm the Lucifer. You know. The, er, the Devil."
Dimpleby raised his eyebrows. Curlene made a sound of distressed sympathy.
[. . . .]
"Stated in its simplest terms," Lucifer said, "the, ah, plane of existence I usually occupy—"
"Hell, you mean," Curl supplied.
"Well, that's another of those loaded terms. It really isn't a bad place at all, you know—"
"But what about it?" Dimpleby prompted. "What about Hell?"
"It's about to be invaded," Lucifer said solemnly. "By alien demons from another world."
[. . . .]
"But why come to me?" Dimpleby said, eyeing Lucifer through the sudsy glass bottom of his ale mug. "I don't know any spells for exorcising demons."
"Professor, I'm out of my depth," Lucifer said earnestly. "The old reliable eye of newt and wart of toad recipes don't faze these alien imps for a moment. Now, I admit, I haven't kept in touch with new developments in science as I should have. But you have, Professor; you're one of the world's foremost authorities on wave mechanics and Planck's law, and all that sort of thing. If anybody can deal with these chaps, you can!"
There is some indication that the Devil is also Prometheus from Greek mythology, the one who gave fire to humans, hence the name Lucifer (Light bringer).
"Mr. Lucifer," Curlene asked, "I hope I'm not being nosy—but how did you get the scar on your side that I saw when you had your shirt off?"
"Oh, ah, that?" Lucifer blushed purple. "Well, it, ah—"
"Probably a liver operation, judging from the location, eh, Lucifer?" Dimpleby said.
"You might call it that," Lucifer said.
"But you shouldn't embarrass people by asking personal questions, Curl," Dimpleby said sternly.
"Yes, dear," Curl said. "Lucifer—I've been wanting to ask you: What did a nice fellow like you do to get kicked out of Heaven?"
"Well, I uh," Lucifer swallowed.
"It was for doing something nice, wasn't it?"
"Well—frankly, I thought it wasn't fair," Lucifer blurted. "I felt sorry for the poor humans, squatting in those damp caves . . ."
"So you brought them fire," Curlene said. "That's why you're called Lucifer."
"You're mixed up, Curl," Dimpleby said. "That was Prometheus. For his pains, he was chained to a rock, and every day a vulture tore out his liver, and every night it grew back . . ."
"But it left a scar," Curlene said, looking meltingly at Lucifer.