I will try to stay with the original Greek mythology, but artistic twist will be applied. Shio's got to go somewhere, so I made her like her role in the game plot.
The Adjustments
"Soma! I heard you finally brought the pretty lady here, is it true?!" Shio asked as she joyfully rode on the middle neck of Cerberus.
"What are you doing neglecting your duty?" Cerberus let out a low whine as it lowered its ears and heads.
"Don't be mad, Soma, I just brought them out to play." Shio defended.
"Whatever, just hurry back to your post." He said rubbing behind each of Cerberus's ears. "We wouldn't want our guest to run away, would we?"
"You mean she's here?!"
"Yes."
"Yay! I want to go play with her!"
"Go on then, find her."
"Okay!" Shio gleefully said jumping off Cerberus, running off into the palace to go meet the Underworld's Queen. Soma smirked at her antics then looked to his pet.
"Shouldn't you be heading back now?"
...
Kore was lost in her thoughts when her door click opened. At first she thought that Soma had returned or a servant had found her. But was mistaken when the head of a little girl popped out from the side of the door to greet her.
"I found you! Hi! My name's Shio!" Kore found her antics to be so cute that she couldn't help but smile at her.
"Aw, you must be Soma's little sister. Since you greeted me I must greet you in return. My name is Kore, nice to meet you."
"Kore... ah, that means maiden!"
"Yes it does. Well done little goddess of the Moon."
"Yay! Hey Kore, are you bored?"
"W-well I am a little."
"Do you want to go play with me?" She would have loved to play with the cute child but she was nervous about exploring the Underworld.
"Well... I don't know Shio..."
"Come on, please." She said giving her the biggest puppy dog eyes, Kore couldn't resist.
"Oh, alright."
"Hurray! Let's go!" Shio said, taking her hand as she ran to the balcony to take flight. Well Kore flew, Shio sort of floated.
"Where are we going, Shio?"
"We're going to get the big doggie to play with us too!"
"Dog? I love dogs."
"Me too!" Taking the chance Kore looked around.
"My this place is so well lit for an Underworld."
"That's cause big brother doesn't like the dark. He has these artificial lights all over the place. They're almost like moonlight."
"I see." Kore pondered. For a guy whose power ruled over darkness, he sure had a fondness for light. She kept looking around until she spotted something that caught her eyes. "Shio, stop for a second."
"What is it?"
'"Look down there."
"I've never seen that there before. Let's go check it out!"
"Right," Kore agreed as they descended down into a flowerbed with grass and trees surrounding it. She couldn't believe it, these were the plants that were sucked down with her. Soma stuck them back into the ground.
"Wow! They're so pretty!" Shio said in awe. "Brother must of went through a lot of trouble to keep them alive. Usually only the tough plants can survive down here."
"Maybe... he isn't so bad after all." Kore said, touched by the fact he cared enough to keep them alive, knowing or unknowingly realizing it would make her happy.
"Hey what's that?!" Kore looked toward where Shio was pointing to see her weapon lying in the bed of flowers, shining like a sunset.
"Fervore!" She cried out in joy, gathering it in her arms. "I thought I lost you, my friend."
"So that's your weapon? It's so pretty!"
"Thank you, she's please to hear you say that." Kore said as it rang out like a song at the complement.
"It's good that you have it, since you'll need it if you want to be playing with the big doggie."
"And what exactly are we playing, Shio?" Kore asked slightly worried. Were they hunting walking corpses? Were they going to torture poor souls for fun? But then Shio giggled and gave her an answer.
"We're going to hunt down Aragami!" Kore sighed in relief, the answer brought her fast thoughts moving toward more morbid scenarios to a halt.
"Oh, that does sound like fun."
"Good! Let's go get the doggie!" So they took flight once again.
...
"Shio..."
"Yes?"
"The doggie is a three headed Marduk." Kore said as the two look each other in the eyes.
"Yes he is!" Shio responded, skipping to the beast.
"Careful." Kore said trying to stop Shio.
"He won't hurt us. He's Soma's doggie!" Shio went to it's side to crawl on his back.
"Soma's?" Kore slowly approached him, noticing how it lowered its body to let Shio on while continuing to make eye contact with her, "So you're Cerberus." It lightly nodded to her.
"You're different than the others. For a creature who looks so dangerous, you have such a gentle aura." She said, slowly placing her hand in front of its middle head. It sniffed her hand while the other heads sniffed her. It seemed to like her since it placed its head in her hand. She gasped in awe at this, this was her first encounter with an Aragami that didn't try to attack her.
She began to pet and rub its head. The other heads began to cuddle to her, begging for the same affection. Not wanting to disappoint, she began to give the other heads the same treatment as she gently rubbed her head on the middle head.
"You really are just a big puppy, aren't you?" She giggled as they began to lick her and wag their tail.
"I knew you would like him! Are you guys ready to play?!"
"Yeah sure, let's play."
...
Soma morphed from shadow to shadow, heading toward where his senses told him Kore was. Oh, how vexed he was at the moment.
Not only did she disobey him by going toward the Gates, but also endangered herself by going to the area where new born Aragami roamed. Even the resident souls wouldn't wonder there. Was she trying to escape? Where was Cerberus?
He made it to the location only to get a surprise he didn't expect. Shio and Kore laughing together as they fought Aragami with Cerberus- who was going through Aragami like a chew toy.
His sister had brought out her luminous short blade to dance alongside Kore's long blade. It almost sounded like the blades were singing together. So that's where her blade was, he wondered where it went when he brought her down here.
Suddenly Shio spotted him and started to wave at him. "Soma! Come play with us!" Kore stopped for a moment to look up at him with a small smile.
"Yes, come and join us."
"You shouldn't be so close to the Gate." Then he looked toward Cerberus. "And you should be guarding it."
"Please don't be mad at him. He's technically doing his job..." Kore said coming to Cerberus's defense. "Nothing is getting pass you, right Cerberus?" He barked in response earning a smirk from Soma. So she was able to win over the beast made from the darkest depths of his mind. She truly was meant to be his Queen.
"So... are you going to come down here or are you going to watch us have all the fun?" She playfully asked shyly, pulling him out of his thoughts.
"Yeah, I could use the break anyway." Summoning his Evil One to his hand, he jumped into the fray with them.
...
"That was so much fun! We made quite a team!" Shio happily exclaimed.
"Yes we sure did." Kore agreed as they sat at the long elegant dinner table.
"What's wrong Kore? Are you not comfortable?" Soma asked.
"No, no. It's not that at all." She says waving her hands. "It's just that I'm use to eating in the forest, this fancy setting is new to me. But I'm getting use to it." Not to mention she was slightly surprised that Soma pushed her chair in for her instead of letting one of the servants do it, like they did for Shio.
Although the table was long and spacious. Soma had it where Shio sat to his near left and Kore sat to his near right as he sat at the head of the table.
"Soma, we should throw a party to celebrate the welcoming of your new Queen!"
Kore began to blush and fidget. It was one thing to deal with the whole kidnapping bride thing but another thing all together to deal with parties. She always hated how they brought out the inner monster in people. The criticizing, the gossiping, the dressing up, the bloody masks that hid them. She hated it all.
"I don't think Kore would feel comfortable with a party." Soma said, noticing her inner turmoil and came to her rescue. Then he grabbed his chin in thought. "Still, you're right. We must have some form announcement to present the Underworld its new Queen."
"What?! But a party's a great idea, isn't it Kore?!"
"... Actually Shio, I don't like parties." Not even Shio's puppy dog eyes could work for this subject.
"But why?"
"..."
"That's enough Shio. The food's here." Soma said, ending the subject as the servants came in with their trays of food.
"Dinner! Dinner!" Shio happily exclaimed as she opened the lid to her plate to get a whiff of her food. "Ah, looks tasty!"
She tried to eat the sweet looking cupcake first, but Kore snatched it away before Shio could reach it.
"You shouldn't eat dessert first, you'll spoil your appetite." She lightly scolded.
"Aw, but that's the best part." Shio pouted.
"The best should be saved for last. It'll give you something to look forward to when you're done eating."
"Okay." Shio started storming through her food.
"Amazing... After all these years I could never get Shio to eat her food first before dessert." Soma said impressed.
"That's because you spoil her like the big brother you are, Soma."
"(Chuckle) I suppose I have." They seemed to have made a connection as their eyes locked.
"I finished my feast!" Shio triumphantly shouted, breaking them from their gaze.
"Good job Shio!" Kore said patting her head and handed Shio back her cupcake. "Here's your favorite part."
"Yay!" Shio cheered at being praised and getting her cupcake back. She took her time eating it, giving Soma and Kore time to catch up with her pace.
"Ah, that was tasty!" Shio then stretched into a long yawn. "What a great day today was. But now I'm ready to sleep. Kore, let's sleep together."
"Sure, that'll be fun. Like the old sleep-get-togethers me and Alisa use to have."
"Hurray! We can brush each others hair and tell stories!"
"I will only tell one story, a growing girl needs her sleep."
"But, I want to hear about your adventures."
"There will be plenty of time for them another night. That'll give you something to look forward to at our next sleep-get-together."
"Okay!" Shio and Kore began to head toward the room Kore was to stay in when Kore noticed that Soma stayed at his seat.
"Soma, aren't you going to bed soon?"
"No, I have a little more work that needs to be done. You go on now, I'll get to bed soon."
"Oh... alright. Well then... good night... Soma."
"Sleep well my Queen." She began to walk in a quicker pace, blushing all the way. This was all new to her, she'd never had a man take interest in her. For a guy that was ruler of the dead he sure was a smooth talker.
...
Soma was exhausted. It seemed that Tsubaki was restless at the disappearance of her daughter. She continuously asked the other gods over the location of her daughter. Soma was safe for now, but he had a lot of explaining to do once they found out about her location.
But he'd be damned if he was going to hand her back. From that one day he spent with her, he felt more alive than he felt in his entire life. He became addicted to her presence, he couldn't live without it.
As he entered his room he looked to his bed to see his Queen embracing Shio in a sisterly hug as they laid in peaceful slumber. He had to smile at the warmness of the moment. Already she had found a way into the heart of his sister, hopefully she could do the same for his parents. She truly did bring color into his world.
Giving into fatigue, he crawled into the bed, embracing his Queen by the waist from behind as he pulled her into him. She was a perfect fit, as if she was meant to be there. This beautiful creature was made for him. He let that thought linger as he fell into the most peaceful sleep he's had in years.