Sinenmehilias

"Look at me like that again and I'll make insulation out of your intestines."

- Sinenmehilias to Issei Hyoudou Sinenmehilias is a unicorn (although he resents the phrase and every other unicorn in existence) who occupies the form of a little girl. He is a creature of great, vindictive rage, possessing frightening powers which no other unicorn has, leftovers from his evolution to his current state.

Currently, he is the familiar of Momo Hanamori

Background
Sinenmehilias was originally an Elemental Spirit, a being closer to a crystalised idea rather than a physical construct, an expression of nature given shape. He expressed the beauty and purity of nature; the quiet moments of clarity in a forest, the blooming of roses, he represented them all, but he was also the beauty of survival, of tearing an enemy to shreds, the dark, twisted beauty of efficiency. During his tenure as such, he was as alien to man as the very earth itself. His own morality was as complex and inhumane as it was simple and perfect. Any attempt at labeling him would have been wasted on a being who existed before such things.

But his existence was closer to a legend, a crystalised idea rather than a physical construct. Due to this, he was suspectible to change on a conceptual level; over the course of history, as man grew to challenge nature, his own existence was challenged. The idea of "a unicorn being the most beautiful being" challenged his position.

It evolved to the point where, instead of "the Elemental Spirit of Beauty", he was something closer to "the Elemental Spirit of Beauty, or a Unicorn". The addendum to his existence slowly became the focus, with his connection to nature being secondary, eroding to the point where his mother could no longer identify or understand him.

This deterioration of his existence continued until the modern legend of the unicorn. Except the unicorn was now less common. But, another aspect of the legend flourished: that of the maiden. His existence now dictated by the whims of humanity, he ceased being a magical horned stead and became a pure, beautiful maiden. With his new level of consciousness came the ability to properly comprehend his existence, which he'd lacked during his tenure in the form of a true unicorn.

And, with the realisation of what he now was - and of what he once was - came rage. A great maelstrom of rage and hate. It was in this state that the Sitri Peerage found Sinenmehilias in the forests of Kuoh. He ceased his rampaging and became fixated on Momo, immediately claiming her as his master. While his reasons for this remain unclear (he simply said that she was "as close as anyone could get to mother these days"), his dedication and loyalty to Momo remain stalwart.

Trivia

 * Were he in any other country on Earth, the chances of Sin manifesting as a virgin girl instead of his regular unicorn form are close to nonexistent. Only in Japan was this possible due to the nature of the culture.
 * Whether his childlike body is also due to this is unknown.