A/N: Hi everyone! This is my first story I've ever written, but I couldn't hold back on this idea that I kept thinking about!

I do not own Blue Exorcist or Fullmetal Alchemist. They belong to Kazue Katō and Hiromu Arakawa.

Anyways, hope you enjoy the story!

04/29/15: This is the first summary that was for this story until it was replaced, so I included it here as reference.

What if on the day Rin and Yukio Okumura were born they had a sister as well? Only instead of being raised by Fujimoto like them Mephisto was given custody of her who sends her to an alternate universe completely different from their own! How would she live her life with the Elrics in Amestris as the daughter of Satan and a state alchemist?

Prologue

"So what will you do now Fujimoto-kun?"

Two men stood inside a cave and faced a young woman resting on a large flower bed. At a farther distance someone would just assume that she was merely sleeping. Her long, chocolate brown hair was a tangled mess and her light skin had begun to lose its radiant warmth and was slowly submitting to the wintry temperature. She had finally died after giving birth to her three children; the ones her arms clung to even after her passing. Two of the babies snuggled closely to each other, burned brightly, bathing the once dark cave with a blue light.

"We were ordered to kill Yuri Egin and her offspring, but it looks like she beat us to the punch." One of the men stood out from the other with his white suit shirt and undershirt, jester style pants and his pointed shoes. What is more unusual is his dark purple hair that's lighter at the center of his head, a piece of his hair that defies gravity and is in a spiral-like shape, pale skin, and forest green eyes. He eagerly waited for his collaborator's response who continued to gaze at the scene in front of him.

This man resembled a priest with his black robe and exorcist pin worn around his neck as an extended necklace. Unlike his younger looking partner, he is a middle-aged tan fellow with crimson colored eyes, explained by his white hair and goatee and several wrinkles that made home on his forehead. The two in general were an odd bunch to be seen lingering in the woods together.

"We will do what we came here to do Mephisto. They are the spawns of Satan after all. It would be cruel to let them live." Fujimoto stated as he came closer with the demon slaying sword Kurikara that he brought with him after receiving his assignment. He was friends with Yuri, worked together as exorcists. They both had their differences however with her believing that demons and humans could one day coexist in the same world and him despising every demon that he ever encounterd. Demons do not belong in Assiah. This includes half-demons as well. These three in front of him, Fujimoto believed, were given a burden that would torture them if they were spared. So it's best to relieve them of that right? Before he could make a move, the oldest child giggled. Fujimoto immediately hesitated. 'A demon child shouldn't act so human.' He thought as the baby smiled and cooed in delight, not showing concern for the sword looming over its body. He remembered once before that Yuri had scolded him for seeing humans and demons differently. Those three are half-demon, yet they act like humans. He sheathed Kurikara.

"Mephisto, I will raise them. I will raise them as human kids."

Mephisto was indeed taken by surprise and not a moment later he threw his head back and chortled.

"You!? The one who is feared for his cold- hardheartedness, raising the children of a demon. HAHAHA! Now there's a sight!" His cackles penetrated the cave's once tense atmosphere.

"Just watch me!" Fujimoto declared, ending Mephisto's mockery, but his amusement was still etched on his face. It is indeed a humorous surprise to hear his friend make the decision to raise the children due to every demon from the lowest rank to the highest quiver in fear even at the mention of his name. In reality, Mephisto couldn't care less about his father's new sons and daughter, his new siblings. Disposing of them would earn him the pleasure to know that he foiled whatever plans Satan had planned. Although, Fujimoto raising them as humans could be something worth watching on whether the two infants who inherited his father's flames would give in to their demonic sides. Automatically he had thought of a wonderful idea!

"Well then my friend, how about we make a bet?" Mephisto saw a new game waiting to unfold and the star pawns were these newborns. Oh how exciting this is!

"If you successfully raise them as humans you win. However, should they expose their demonic powers they would then belong to me."

Seeing no fault in this Fujimoto agreed. His partner snapped his fingers and the blue flames that had engulfed the oldest son were sealed into the sword. Now the sister of the two brothers is the only one illuminating the obscure cave. Fujimoto lifted the sword in the air to see the flames dance on the sharp blade. "What did you do?" He asked.

"I've sealed the boy's powers into the sword since it wouldn't be fair if he's always covered in blue flames."

"Thank you." It was then that he noted that Mephisto did not seal the girl's powers as well, but his friend had concocted another scheme. Giving Fujimoto the chance to raise the infants as humans is one thing, but what if he had one of them for his own show? One where he had a new board to place pawns of his own and achieve new insights from the game at a leisurely pace. Having detected that Fujimoto eyed him questionably, he decided that it was time to reveal the second part of his plan.

"Now before I go I do have one more request to ask of you." Boy did Fujimoto not like the sound of that.

"I would like to have custody of the middle child. It would be fun to have a part in this game after all..."

For as long as he's been friends with Mephisto for a long time and he knew that he is someone whose actions are always suspicious. For that he grew a little alarmed at his sudden decision to take the girl.

"Why are you so interested in her?" Fujimoto pressed.

"This does seem unusual for me to ask of this but I believe that it's best to separate her from her brothers. Satan will surely be after them and it would be much more difficult for him to get what he wants if he has more than one place to search."

That was just one of the reasons for him wanting the girl because he could somehow see the promise that little girl has! Of course her brothers indeed displayed the same intoxicating guarantee for greatness, but the girl's much more...intriguing. Never having a little sister could possibly have been the most elaborate reason but he didn't care about that so long as she was his. He really wanted to see how much more powerful would she become compared to all her brothers.

Fujimoto thought this through carefully. He wanted to raise all three of them, but what Mephisto had said was indeed accurate. Separating the two that acquired their demonic flames would keep them safe...and he didn't want to risk refusing Mephisto and lose the chance of keeping her brothers.

"Will she be able to reunite with her brothers in the future?" If he couldn't have the daughter, then he just needed that one final confirmation. One which his friend delivered graciously.

"Shiro my friend, I can assure you of her safety and that they will indeed meet her again."

That was enough for Fujimoto to submit. He reached into the dead hands of the mother and gathered the child into his arms. She cried with the uncomfortable feeling of being removed from the warmth she had received from her mother. He smiled warmly at the small bundle he carried, then gently placed her into the patient arms of Mephisto. The child somehow managed to relax and returned to her silent slumber, unaware of her separation from her brothers.

"Well then," Mephisto turned around and started to walk towards the cave's entrance.

"I will inform the Vatican that we have completed the mission. Of course after I hide the girl."

With that he left Fujimoto alone, taking the one who had brightly lit the cave and leaving him in the dark with his newly adopted sons. He removed his pack of cigarettes and lighter from his lower left robe pocket. He flicked the lighter on to ignite the cigarette, but stopped when the baby laughed once more. He dropped his pack to the ground and firmly pressed his shoe on top of it. He wouldn't smoke. He would start fresh for his newly adopted sons. With that, he disposed of the lighter as well, cradled the children in his arms and left the cave.

Walking out of there, he felt like the twins were incomplete without their sister, who they wouldn't meet for who knows how long. "Don't worry. You will meet her again. I promise." That's all he could do, make a promise that he would surely keep to them.

With that he traveled through the white blanket of snow, the cave disappearing behind him.

OoOoOoO

Mephisto snuck into his office to make sure no one knew of his return. He cradled the child carefully, so she would not be bothered from her nap. Her flames remained lit around her but they didn't burn to the touch, no rather, it felt warm. He wondered why would his father's flames burn and yet hers would not but he assumed that they were most likely weak at the moment and not controlled to.

It was indeed upsetting to see Yuri die after delivering her children into the world, but Mephisto inwardly thanked her for allowing him to watch an exciting new game to transpire. One would believe that he needed time to think on where to hide the little girl, however, the perfect location was already set in his mind when he gained the idea of guardianship and it all pointed towards that experiment he did earlier.

Months ago, he decided to test the limits of his powers as the demon king of time and space. It was just a theory, but he had wondered if he could actually create a portal to another dimension. Sure he had constructed other dimensions before, however, those were just endless voids of darkness. A waste of his time. So with having nothing to do at that moment and with his curiosity constantly pestering him, Mephisto decided to have one more go at it. This time, using all the energy he could muster. He opened a door to a closet holding several school supplies used for teaching exorcists in training. Surely they wouldn't be missed if anything goes wrong.

His "little" experiment absorbed vast amounts of his strength. He nearly quit when he reached a certain limit until a miracle happened. The portal he opened shifted from black to white, having obvious difficulty to return to its once original form that had consisted of nothing. Suddenly, it unraveled and revealed an entrance to an entirely new world! He had done it! Created a passage to an alternate dimension! Mephisto entered to admire his masterpiece and discovered that it completely contrasted with Assiah from his dimension.

'This world's more...old fashioned.' A fact that hardly bothered him at all for Mephisto assumed that there were traits from this world which were not in his own. Sadly, he couldn't research more due to his important role as an exorcist and as principal of True Cross Academy but most importantly, he refused to abandon his favorite playground!

That's why now, he would send the newly born infant there in his place. She would grow up there and learn their culture, their history, everything...and once she returned to him, Mephisto would achieve the knowledge from her. Oh it was a delightful plan indeed!

And so without further ado, he pulled out a gold metal key, identical to the ones every exorcist receives to travel around True Cross or to the Vatican's Japanese Headquarters. Though this one only led to that alternate dimension. He inserted it into the lock of his office door and entered the portal with the girl still sound asleep.

They entered through a house that resides on a hill and is separated from the other homes by a couple of acres of land. The day had already gone by in this small rural town and the night has taken charge. Despite that, the white two-story house could be clearly seen with its front door and its five windows coated olive drab and the roof a shade of scarlet red. A tree stood nearby with a wooden swing attached to it by an old yet sturdy set of ropes . 'This is the ideal family home' Mephisto mused.

Mephisto carefully placed the precious bundle on the front porch. Everything was set in place, but he needed to somehow leave evidence of her name. Now what was it again? He tried to remember what Yuri named each of her children. The oldest born with the flames like his sister is Rin, the youngest who was too weak to acquire demonic powers is Yukio, and the girl...Ah! Mephisto removed a slip of paper and a pen from his shirt pocket and wrote down the girl's name. He could just allow her to be renamed by this family, but one name given by your birth mother is always the best treasure to hold close and dear to your heart.

"And now the grand finale." He smiled as he twirled his hand in the air. "Eins, zwei, drei!"

A small crystal orb appeared in his hand and the child's flames traveled to it and were sealed just like Rin's. The only difference that he made especially for her was that should she ever destroy the seal the orb would break, notifying him of the sudden change of course. Also, once the seal is removed, she gains her demonic powers back and would have no chance of returning to her human state, while Rin would have to have his sword destroyed for his demon half to take over.

It may seem unfair to the common people but to Mephisto it was the price to pay for not everything is free! Everything comes at a price.

He teleported further away from the site and watched the scene play out. The baby began to cry once again, and louder this time. The change in temperature did not seem to please her one bit. Her wails interrupted the night like a rooster crowing early in the morning. The lights of the house turned on and two figures emerged from the front door. A young woman and her husband eyed the weeping child that was by their feet. The woman immediately acted and gathered her up from the floor. They noticed a piece of shredded paper safely tucked in her blanket; the one that had her name written. After conversing for several minutes the couple finally returned inside with the newborn in their possession.

Satisfied, Mephisto began walking away in search of another door to use to return to Assiah with. He smiled with content, knowing that this child will be wonderful to watch grow.

"Auf wiedersehen," Mephisto commented as he embarked on his departure. "Until we meet again...Leeza."

Thanks so much for reading!