I don't know who decided Tails is a girl, but that makes no sense. Obviously, Tails is a guy. Of course, sometimes things happen...
TAILS' SECRET POWER
Tails sighed as his plane limped home from the latest battle with Eggman. He had sat out much of the fight after the mad scientist's robot had managed to hit the X Tornado. He'd watched, pretty much helpless, and Sonic spun dashed the robot, Amy banged it with her pico-pico hammer, Knuckles smashed its head in with his big spiked gloves, and even little Cream and Cheese had contributed more than he had. All Tails had going for him was his ability to build and repair machinery, and his one and only super power, flight. He wished he could be more helpful to his friends.
Tails chuckled mirthlessly as he thought about what Amy had suggested more than once. That Tails was a kitsune youkai, a supernatural fox spirit. Yeah, right! If he were a youkai, he'd be able to run in the air, chaos control, breath fire, and create illusions, and Tails had never seen even the slightest sign of any of those abilities. Heck, he couldn't even became a human, which would have been helpful while they were on Earth before their existence became common knowledge.
Actually, it suddenly occurred to him, he had never really tried to use any of those powers. He didn't expect any of them to work, but what was the harm in just trying them in private?
Tails stepped outside. He started running. Faster and faster. Then he leaped into the air, still running. "Ack!" Crash! "Oof! Ow!"
Okay! That sure didn't work! After putting peroxide and bandages on his scratches, though, he went into the kitchen, deciding to try something less dangerous. To breath fire, he needed hot breath. He picked up a bottle of hot sauce. "Extra Hot Call the Fire Department!" it said on the bottle. He unscrewed the cap, then took a sip.
"Argh! Ahh! Bleck! Fire! Fire! Hot hot hot!!!" He turned on the kitchen faucet and stuck his head under the spigot. When he finished, he checked in the bathroom mirror.
"Oh, great! I burned my tongue! I oughtta have my head examined!"
"I should stop now before something worse happens," Tails thought. But it would be great to have some kind of extra power to help his friends. He decided to try chaos control next. He had a fake emerald Eggman had made that he had been experimenting with. He went to his lab, picked up the emerald, shut his eyes, and concentrated.
"Chaos control!" he said. Nothing happened. "Chaos control!" Still nothing. He concentrated until he could feel perspiration running down his forehead. Then POW! The fake emerald blew up in his hand. It dissolved into a pile of grey powder.
"Ahh! My hand!"
After applying burn ointment and bandages to his hand, Tails decided he might as well try the illusions. Surely nothing could happen to him there?
Tails sat on the floor and assumed the lotus position like he'd often seen Knuckles and Espio use. "Come on!" he said. "I want to see…a rock!" Nothing. "A flower!" Nothing. "A one-eyed, one-horned, flying, purple-people eater!" Still nothing.
Tails got up. "Phooey," he said. "Nope. No new powers. The only thing left is transforming to human form. Oh, why not?" He clenched his fists. "I'm a human!" he whispered, trying to picture himself looking something like Chris. "I'm a human!" He saw Danny, Frances, Helen, Grandpa Chuck, Mr. Tanaka, Ella. "I'm a human!" he repeated. "I AM A HUMAN!!!"
POOF! There was a cloud of dust.
When the dust settled, Tails looked at his hands. His arms. They were human! He'd done it! He couldn't think of how that power would be helpful in a fight, but it was good to know that he could do something besides…wait a minute! "Why am I wearing a dr…Hey! What's the matter with my voice?!?" Gasping, Tails ran to the nearest mirror.
A human looked back at him. A human with blue eyes and curly blonde hair, wearing a yellow dress.
"Oh no!" Tails cried in a high-pitched voice. "I'm a girl!!!" Then he remembered what Amy had said. "For some reason, in the legends, many animal spirits transform into a human of the opposite gender from their own. Nobody knows why, but it's in a lot of stories."
"No!" Girl Tails cried. "I'm a boy!" He tried to change into a human boy. Tried again and again. Finally, he gave up, and willed himself back to being a fox, praying he'd be a boy in his natural form. He was incredilbly relieved to find that it worked. But he was never able to become a human boy. And he was afraid that he might get stuck as a girl sometime, so he swore to never use that power again.
And that was why Tails never told anybody about that power until…but then, that's another story.
THE END