If you just want the smut, skip all but the last paragraph of this.
Stories. Cinder had always heard stories of the mountains behind Beacon. Stories of a monster, slumbering inside. Stories that said monster could create Grimm simply through its presence. Stories warning that the awakening of the creature would bring destruction upon the entirety of humanity and faunus. Stories that no one really believed, but told anyway, to scare their children in line.
Stories that were not fiction.
Such stories seemed to be common in Remnant, thought Cinder, as she hiked her way to the infamous mountain. She had only recently learned that the Maiden stories were true. The same "person" who told her this also told her about many similar stories that were not as fictional as they seemed. Cinder would not have believed them . . . except that Salem had irrefutable proof. She shuddered, remembering the encounter.
Turning back to the task at hand, Cinder channeled all her aura into a single attack, slamming her fist into the side of the mountain. She half expected nothing to happen, the rock dissipating her attack with no damage. That would be the best indication that Salem was wrong about this particular tale. Instead, the rock shattered inwards, leaving a hole wide enough for Cinder to easily walk through. Inside, she noted a lack of . . . well anything, really. There weren't armies of Grimm waiting, having been created by the monster. In fact, the monster was nowhere in sight. She cautiously lit a match, better illuminating her surroundings.
The only thing the light shone upon was a large, empty cavern, with a series of slightly smaller than human sized tunnels. She sighed, slightly relieved that she would not have to face the monster of legends. There was a cavern here, but it was uninhabited.
She turned to leave, but quickly froze, hearing the sound of rapid clicking. It was deafening. She turned back to face the cavern, but couldn't tell the origin of the sounds. It was echoing everywhere, seeming to come from every direction at once. Panicking, she turned to run out of the hole she had entered through, only to come face to face with some creature of Grimm, already pouncing on her. She only had time to bring her arms to her face before it knocked her over, pinning her down.
This is the end. After I've learned so much, created so many plans. This is how I die.
Cinder couldn't protect herself. She had used all her aura blasting her way in, expecting to find a slumbering giant. There was no way to escape. There was no time to think of a plan.
But there was time. Cinder seemed to have all the time in the world.
Why isn't it devouring me?
Cinder hesitantly opened her eyes, seeing a small dragon-like Grimm perched on her chest, staring at her with its' head cocked like an inquisitive dog. It was unlike any Grimm Cinder had seen before. It was only the size of a large dog, and seemed to have no intention of killing her.
Cinder layed there for another thirty seconds before deciding she needed to do something. She tried to push the Grimm off, but when she started moving her arms it growled, a deep, throaty growl and let out a small roar. Along with the roar came a puff of flame, enveloping Cinder's head.
The fire went out as quickly as it came, having no effect on Cinder. She had chosen the name herself, after learning of her powers. Before she was always referred to as "orphan" or simply "you there."
The Grimm was obviously surprised that his fire hadn't caused her to cry out in pain, as it had every other creature that ventured into one of his tunnels. He jumped off of Cinder and circled her,
head still cocked inquisitively. Cinder slowly got up. She couldn't decide what to do, so she held out her hand, feeling stupid. Grimm weren't dogs. They had no thoughts. No emotions. No s - her thoughts were cut off as she felt a hot breath on her hand.
She spent the next few days camped nearby, slowly training the creature to trust her. She decided to start calling it Kevin. The more she tried to train Kevin, the more he acted like a dog. He would bring her immolated creatures, leaving them in front of her tent and staring at her, waiting for her approval. He would fly around, attacking anything that got close to her, even other Grimm. He would land on her shoulders and sway his tail in excitement, almost overbalancing her with every swing.
One day as she was foraging for firewood she heard Kevin sprinting towards her from behind. She looked up, expecting a Beuwolf or something. Seeing nothing, she was confused at what Kevin was charging. Then she felt him barrel directly in between her legs, knocking her onto his back. He spread his wings and took off. They were airborne.
If you want more cute stuff like this, don't continue. Read something else. Anything else.