A Railgun In Fairy Tail, The Diablo Saga, Book 5: Inner Darkness.

The Cutest Fusion of All Time

(Hint: There will be references to plenty more Railgun Crossover Series I plan on starting next month.)

"I'VE GOT IT!!!" Mikoto jumped to her feet.. as Natsu and Luffy both gave a start.

They were still in  Yuriko's house, and they hadn't even heard about the incident at the hospital yet..

Usopp gave Mikoto a tired stare. "What?"

"I know how to do it!! To beat my dark self!" said Mikoto. "It was so simple! All this time, and I didn't even think about using God Fusion!!"

"We did use that!" said Natsu. "Thundamander got smoked remember?"

"Yeah.. but she's ME remember? She has MY weaknesses, even if she is a more powerful version of me." said Mikoto. "And fact is, she, like me is way too dependent on her powers.. rubber is immune to electricity!!"

"Wait.. you wanna fuse with Luffy?" Natsu said. "Heh.. sounds like it might be interesting.."

"Fuse?" said Luffy, tilting his head to the side. "Auuughh? What's that?'

"It's called God Fusion." said Mikoto. "When somebody like me.. who has Numensapien Origins uses what remains of her power, I can fuse with other people, and combine our powers to reach God-like levels."

"Sounds kind of dangerous." said Usopp.

"And, anybody I fuse with once, will also gain the power to fuse with others." said Mikoto. "I admit, you're right Usopp, there's no telling how the fusion will turn out.. boy, girl, psycho, weirdo.. unless I've done the fusion before, there's no way to see what the fusion might be like.."

"But the other you kept on melting my rubber." Luffy said. "So annoying.. made me so hungry."

Fusing actually strengthens our original powers beyond ordinary levels while fusing our powers at the same time, your rubber would have a much higher melting point." said Mikoto. "Come on, it's worth a try right?"

Luffy grinned. "Okay!"

"WHAT!?" Usopp stammered. "Come on Luffy!! You're just going to jump right in without considering the risks!!?"

"Mikoto is my friend." said Luffy. "That's good enough for me!"

"Well said!!" said Natsu, sharing a grin with Luffy.

"Oh what's the point?" Usopp muttered. "I'm sitting here with two hungry risk takers!!"

"Alright.." MIkoto said as she put hand on Luffy's shoulder. "Just stay calm.. this might feel a little weird at first, combining our consciousness into one thought and being.. but we'll get the hang of it."

Mikoto closed her eyes and took a deep breath. "God.. FUSION!!!!"

"BOOOOOMMMMMM!!!!" There was a sudden shockwave.. and light exploded throughout the entire house.

"WHOOOOOAAAAA!!!" Luffy yelled as he felt his mind meld with Mikoto's. "THIS IS SOOOOO COOOOOOOOOOOOLLLLLLLL!!!!!!"

When the light cleared. A girl stood  in the middle of the room...

She possessed a similar red coat to Luffy's that was cut off around her midriff, and black hair that was styled in similar way to both Luffy's and Mikoto's

Luffy's scar had moved to the girl's belly, just over her belly button, and she wore a skirt that bore a resemblance to both Luffy's pants and MIkoto's own skirt.  and upon her head she wore a large straw hat, with a small scar beneath her left eye.

Rubbergun grinned widely, small features of Luffy visible within her face as well as Mikoto's.

"WWWWWWWOOOWWW!!!" Rubbergun said, looking at herself and flexing her arm. "Heeheeheehee! So cool!!!! I'm like flexible now! Or.. wait.. have I always been flexible! Ehee! Also true!! This is awesome Mikoto!! I mean Luffy.. auuuughhhh this is confusing.. talking to myself.. or myselves.."