I have no information past vol. 6 of the light novels and this is a divergence AU regardless, although I do hope that nothing revealed later makes this end up too ridiculous in a bad way. There's only one person trying to sink ships here, and it's not the author.
This is dedicated to Kyogre, who I hope finds it amusing.
I'm using Lailah instead of Layla here because Lailah is an actual angel name, like Gabriel and Sariel. Or Layla could just be the name she uses when she's posing as a human.
And thus it was that Chiho, Demon-General and future Great Sage of Ente Isla, her wisdom distributed to the people by her first adherent the former Inquisitor Christia Bell, paused while eating dinner in the Devil's Castle.
"Maou-san... you said that Lailah took care of you after your parents died..." When Maou nodded for her to go on, she continued, "Doesn't that make her kind of your step-mother?"
While everyone was processing this, that the Demon Overlord and Assistant Manager of the Hatagaya MgRonald's was the step-son of an archangel, the teenager moved in for the kill, smiling cheerfully. "And if Lailah-san is your step-mother, wouldn't that make Emi your step-sister?"
The Hero Emilia had been taking a drink of barley tea. In shock, she spat it out, unknowingly short-circuiting Ashiya's attempt to tactfully tell Sasaki that she was wrong, and his liege was in no way related to that, that hero as he instead winced at his shirt and grabbed a napkin to wipe the table.
Alas Ramus, sitting in Emi's lap and holding her sippy cup, blinked in incomprehension.
Ignoring Ashiya's tirade against Emi's un-heroic behavior and Lucifer's, "Hey, not in front of the kid!" as old angelic instinct made the First Class Bum take action to protect the innocence of the piece of the Sephiroth, Maou first stared at Chiho, then blinked, then looked thoughtful. "Huh," he said slowly beginning to smile. "You might have something there, Chiho."
While on the outside she continued to smile, eyes alight with innocent curiosity, inside Chiho went "Yes!"the way she had when she managed to get them to teach her to use holy force by saying she would totally only use it to call for help, she promised.
"What? Even if my irresponsible mother spent all that time raising a demon child when she couldn't be bothered to even introduce herself to her own daughter, how dare you insinuate that I'm related to a demon!" Emi yelled at Maou instead of the deliberately adorable Chiho. When she started shaking her fist in his direction Urushihara, who had gotten up from his computer for once, reached down and pulled Alas Ramus out of the line of fire.
"Come on, want to play some Dario on my C4D?" he asked her, as part of his plant to make Ashiya shut up about him illicitly buying video games on Jungle dot com.
"Kitty Dario!" Alas Ramus cheered as Maou leaned around the corner of the table towards Emi.
"Play along!" the demon who unified his people said quietly, glancing at Alas Ramus to be sure she was distracted by the bright colors. He and the others were carefully monitoring her exposure to colorful things after what happened at the park, but with Urushihara looking over her shoulder, he should put a stop to anything that might remind her of the other sephiroth before she freaked out like that again.
"Play along?"Emi demanded. "I know you don't care about anyone but yourself, but Emeralda and Albert are already having enough trouble without someonestarting a rumor that I'm related to a demon!" If the church could claim that was where she got her powers instead of from the angel mother that had never been around when Emi was a kid? And if no one had seen her, at least until she started freeloading at Emeralda's sometimes, then it would be easy to claim she'd never existed at all, and was just made up to cover-up the truth...
"Do you wantAlas Ramus to keep calling us her parents?" Maou said, effortlessly putting him off-balance with the political skills that let him unify the demon world… the ones Emi's mother had taught him but not her daughter…
That made Emilia pause, because people thinking that she'd... her cheeks started to burn with shame at the thought of... ew, with that devil.
"Alas Ramus wants us to stick together and be friends too much to accept that we don't have anything to do with each other," he continued.
From the grimace on Emi's face, she was well aware of that thanks to her multiple failures to make Alas Ramus stop calling them her parents, even though it was hard to object to Alas Ramus calling her 'Mommy!'
"I'm the one who took care of her when she was a plant, so I don't mind her calling me daddy, but if I'm related to Lailah or whatever, then you're Alas Ramus' aunt."
A vision of Alas Ramus calling her "Auntemi!" flashed through Emilia's mind. Her scowl deepened. "I hate to admit it, but you have a point." Alas Ramus was Emilia's holy sword – she didn't want to make her feel like Emi didn't care about her. After being abandoned by one parent and told she'd lost another, Emi had to sympathize with her not wanting her 'family' taken away. Maybe having a word to call Emi that didn't imply she'd done that with him would let Alas Ramus feel secure and happy enough to go with it? "Alright, I'll go along with your scheme. For now. If you make Alas Ramus cry…"
"That was a masterful tactical move," Crestia Bel said, kneeling down to Chiho and taking the girl's hands in hers admiringly as Chiho blushed.
"I agree," Ashiya said, not noticing Suzuno's glare as he interrupted their moment. "This is why I'm so grateful that you've joined me in supporting my lord as one of his demon generals."
"Thank you, Ashiya-senpai!" Chiho sparkled with delight.
"Ashiya… senpai?" he asked her.
"Well, you are the founder of Maou's fan club, right?" Chiho smiled at him. "Let's do our best, for Maou-san and his dream!"
Ashiya flinched back dramatically, hand over his heart. "Sasaki-san," he managed to say, wiping a tear from his eye. "When we finally return to Ente Isla in triumph, with all our lord has learned of industry, infrastructure, and how to harness the evil power that controls this world, he has already asked me to consider how to 'pointify' his army. Even if you are not able to come with us," because of the importance of her schooling – Alas Ramus was his lord's heir, and would need the best tutors in this world's arcane secrets like 'economics' and 'calculus,' "I will send you one, no, three, of every piece of merchandise we manufacture glorifying our king and his reign!"
Chiho's eyes widened. "Maou… merchandise?" Would there be figures? Of course there would, this was Ashiya and he spent so much time at the library, of course he would have looked into common kinds of character merchandise and how much people would pay for them. Maou bento boxes?
Maou… Maou dakimakura?
Pillow cases for huggable body pillows with Maou in his MgRonald's uniform, and his demon king raiment, and his real demon form of course with all the muscles… "I'm going to need to apply for more hours…" she said faintly. All of her money was going to belong to Ashiya, because three was not enough! Absolutely not!
Seeing Chiho about to swoon, Suzuno handed her a glass of cold barley tea.
When asked, years later, if he would be willing to consider Chiho for his personal savior, Ashiya said that Chiho had been the savior of the household budget so he had every confidence in her ability to save souls, would peruse the pamphlet he'd been offered thoroughly and would like to subscribe to Suzuno's newsletter.
Reading through, I've kind of noticed that Suzuno is very admiring of Chiho, and while there's no indication so far that Suzuno or Emi find men appealing, both of them have checked Chiho out on multiple occasions…
When Alas Ramus called Emi 'mommy' it was because she mistook her for Lailah.
I doubt I will be continuing this particular fic – while I have a few more bunnies for the series vol. 7 should be out soon, and I'll want to do those bunnies with the most up-to-the-minute info I have.
Then again, a lot depends on the personality of Emilia's father. Like, is he going to be just slightly annoyed with Layla and go 'why didn't you tell me that I had a son, I married you so he's mine now too' and is he going to make Maou do father-son bonding exercises so Layla realizes that she's obligated to do mother-daughter stuff with Emi, that kind of thing.
Also the thought of someone seeing Maou and Emi bickering and 'ah yes they definitely are siblings.'
I have a friend whose kid – who acts around Alas Ramus' stage – loves Mario and the kitty transform in this WiiU game. I do RPGs not platformers, so I have no idea if there's an equivalently cute game on the ~3DS, but this is a universe where it's MgRonald's not McDonald's, so.