The next morning.
"Hi Natsu," said Rias.
The dragon stirred and opened one of it's eyes to look at her.
"I'm afraid I have to leave and go to School today," But after seeing the dragon's disheartened look, quickly replied "But don't worry because this is the underworld having your familiar with you is compulsory, which means that you can come with me,"
After hearing this it was easy to see how quickly Natsu had cheered up as his tail was swaying side to side. During their conversation Sirzechs and the others had walked over. Then Sirzechs said "Please Natsu can you look after her?"
Natsu just made a sign to show his agreement. "Very well then, you are free to go, be good for Natsu Rias," said Venelana.
Rias just nodded her head.
"Bye mom, dad, brother and Grayfia," she screamed out as she jumped onto Natsu's back as he prepared to take off.
They shouted back to her and waved as Natsu steadily rose into the sky.
"Let's go Natsu, school is west from her."
As they got closer to the school Rias thought of a plan to prank everyone at school. She leaned down and whispered her plan into Natsu's ear.
Natsu then quickly landed close to the school and Rias quickly got off to explain her plan better. "When I get into school I want you to listen in on where I am, then I want you to roar and then land outside and look into the classroom I am in."
Natsu gave a rumble in agreement. Then Rias left to school.
When everyone was inside the school the headteacher said "Now, now children before we start the day I would like to warn you that there has been a possible dragon sighting nearby, so I would like you all to take extra care while being outside this week. All of you are dismissed and can go to your classrooms." Then everyone got up to leave Rias quickly talked to Natsu via telepathy while everyone else was discussing about the dragon.
Everyone was sat in the classroom while showing off their familiars to the others. When it came to Rias's turn before she could open her mouth Riser shouted out, "I bet she has a really lame one if she even has one at all." Rias glared at him and before she could respond there came a terrifying ROAR and there was a loud thud outside.
Opening the doors all of the devils gasped in shock as standing before them was a ginormous black and blue dragon.
"That's Acnologia run for it!" The dragon glared down at them and opened its mouth and gathered energy inside its mouth ready to blast them into oblivion. All of the devils started crying and screaming when suddenly another dragon this time red lunged at the other dragon and sent it flying into the nearby forest and quickly flew after it, breathing fire at the dragon of the apocalypse. The ball of fire hit the dragon causing a nuclear explosion dust cloud, which sent a massive shock wave flying in all directions, knocking everything in the nearby vicinity onto the ground. However it just missed the gathering devils and the school.
Once the dust had settled it could be seen that nothing remained of the once infamous dragon, not even ashes. The red dragon flew down towards them and landed in front of the devils and roared into the sky in triumph. All the devils gazed up at it in shock at seeing it one shot the most powerful dragon in the young devils that didn't have a familiar all ran up to asking it whether it would be their familiar. However it just ignored them to focus on Rias to the shock and confusion to the it was Rias who said something that blew everyone away, "Everyone I would like you to meet my familiar the new most powerful dragon in the underworld." Everyone's jaws dropped in sync. After Riser had recovered he started spluttering angrily, "That's the dragon that I tried with and it chose you over me?!" Then without warning the dragon got up on its hind legs and roared right in Riser's face. Riser screamed and secretly wet himself. Everyone started laughing at how one of the sons of a high class devil was screaming like a little girl. Even Natsu started laughing in his own draconic way. Riser red from embarrassment and anger turned around and stormed calming down everyone went back inside and continued with the day's lessons.
When Rias got home she was greeted by her overly concerned parents and brother, "We heard what happened at school today, thank Maou you're all right."
"I'm fine" replied Rias in her normal cheery voice, "Natsu protected me, and he killed Acnologia the apocalypse dragon in one shot, it was so awesome," she said.
"It was a good thing that she found him then otherwise I don't know what would have happened to us if something happened to you." Said lord Gremory in a relieved tone. Walking up to the dragon he said "Thank you Natsu, you have our gratitude for you saving Rias." Lord Gremory said.
Natsu just nodded as he closed his eyes in thought.
"Alright Rias, you can go and play with your familiar but make sure to stay within the grounds as we don't want to have a repeat of what has happened now do we?" Lady Gremory said.
"Of course Mother, I'll stay with Natsu at all times, hey Natsu do you want to play with me?" She asked as she ran up to him looking up at his head for a response. Natsu lowered his large head down to her and gave her his best attempt at a smile. Rias' mood immediately picked up at the sign of his agreement. So that's what they'd do everyday after Rias got back from school.
Five years later:
"I'm sorry Natsu, but I have to go to school in the human world and you can't come with me as you're too large and the sight of a dragon would cause a mass panic and then you'd be hunted and people would try to kill you, then you'd fight back and there would be a massive death toll, then you'd be classed as a stray and would have hundreds of devils trying to kill you to increase their fame. If you died I wouldn't forgive you for leaving me!" She exclaimed with tears in her eyes. Natsu wrapped his tail around her waist and sat her down on his snout so he could look at her, looking softly into her eyes and gave a comforting humming sound so to soothe her as she hugged his snout, with tears flowing down her cheeks. After several minutes of being in that position she broke off from the hug and climbed down and said, "It's time for me to go, don't worry I'll come back for the weekends and the holidays and Grayfia can look after you while I'm gone." After a few minutes of convincing Natsu finally relented and lay down on his front legs and just watched as Rias left.
10 minutes after Rias left to go to the human world, Grayfia walked up to the sulking dragon and patted the side of his face in comfort. They both stayed like that for a few minutes before Grayfia spoke to him, as she said, "Don't worry too much she'll be fine, she'll have friends to look after her," then after slight hesitation she again said "You're a very strange dragon you know, you have a lot of human like qualities even though you're a multi tonne fire breathing flying reptile." She said in amusement at watching the dragon sulk as it's tail twitched impatiently.