Nightfall Menace

Nightfall Menace, also known as Colourless Shadow Commander is the Ancient Gear of Miyu Oshiro.

Summary
First introducing herself as Miyu's partner, Nightfall Menace using her alias Nightingale (ナイチンゲールちゃん, Naichingēru-chan), presented herself as a carefree, nonchalant and easy-going, if not a bit strict and sarcastic with her partner and user, young woman who wouldn't leave her partner's side, earning Tobio's and the rest of Slash/Dog team's curiosity and interest once they felt she wasn't human.

It is later revealed that Nightingale is in fact an independent-type Ancient Gear with intelligence and adaptation on par with humans, and with the exclusive ability to shapeshift into any animal either her or her partner want, not to mention the ability to mentally communicate with one another.

Appearance
Being an Ancient Gear with the ability to shapeshift, Nightfall Menace has no affixed appearance or shape, always with the ability to change them on the whim. Under Miyu's possession, it takes the shape of a dark-haired, pale young woman of red eyes and wearing a winter school uniform to reflect on its user's "unusual interests".

Abilities
Nightfall Menace possesses the same level of strength and intelligence of its user, making it relatively smarter than any other independent-type and with the ability to communicate its thoughts and feelings much easier with other creatures. It also serves as a guardian spirit of the user, being able to cover all the blind spots around the duo and react just as fast. When not in use, Nightfall Menace can withdraw into the user's shadow for later use.

Nightfall Menace's main ability is the ability to shapeshift and transform itself into any living being, being vegetal, animal and even human as seem when taking its default form. Under the user's command or itself, Nightfall Menace turns itself into a black sphere of darkness and after some period of time, a new creature emerges from it, with all the signature abilities of the chosen animal (example: A lynx's speed, a hound's ferocity, etc.) while maintaining the same level of intelligence and memory. Both Miyuu and Nightingale can also share information with one another through a mental link between the two, and using her magical knowledge, she can channel elements into it for a more devastating effect.

Forms
A shapeshifting Sacred Gear, Nightfall Menace can turn into a myriad of animals under the user's command:
 * Default: Nightingale (デフォルト：ナイチンゲールちゃん, Deforuto: Naichingēru-chan): Nightfall Menace's default appearance in the shape of a young woman wearing a winter school uniform. This shape has no weaknesses or strengths and is capable of communicating with Humans, Vampyres, Devils, Seraphs and other supernatural creatures alike. Its default form for presentation.
 * Cat-Cat Change: Black Panther (ネコネコチェンジ：ブラック・パンサー, Neko-Neko chenji: Burakkupansā): Nightfall Menace's feline transformation, it takes the shape of a large black panther with matching speed, senses and with abilities of a natural hunter, being able to easily find and prey on foes with its sharp fangs and claws. The fastest transformation, but also the weakest.
 * Dog-Dog Change: Black Hound (イヌイヌチェンジ：ブラック・ハウンド, Inu-Inu chenji: Burakkuhaundo): Nightfall Menace's canine transformation, it takes the shape of a large and intimidating red-eyed hound with sharp teeth and razor-sharp dark fur, with unmatchable sense of smell and an even higher level of savagery, being a powerful and merciless killer. The Strongest Transformation, but with no commentable speed or reflexes.
 * Bird-Bird Change: Black Hawk (トリトリシェンジ：ブラック・ホーク, Tori-Tori chenji: Burakkuhōku): Nightfall Menace's avian Transformation, it takes the shape of a gigantic black falcon with strong red talons and sharp claws, with acute sight beyond any other animal and high-speed flight, making it perfect for espionage and patrolling. The most versatile transformation, but also the most fragile and vulnerable.
 * Wood-Wood Change: Black Forest (ウッドウッドチェンジ：ブラック・トリー, Uddo-Uddo chenji: Burakku torī): Nightfall Menace's vegetal Transformation, it turns itself into a large and bulky black tree which resembles several smaller trees tangled together to form a bigger one. With the user as its center, this transformation is a defensive one which cannot be cut, broken or even burnt unless by very powerful magic or attacks, even by Tobio's attacks and Damian's flames. The defensive transformation, but it can't move or communicate.

Trivia

 * According to Miyu, Nightingale's chose her own nickname, and her favourite foods seems to be cheap candy and junk food.
 * Nightfall Menace apparently can communicate with other Ancient Gears, even the ones with no conscious or beasts sealed inside of them, and give hints for the user of how to better control them.
 * She's named after Nightingale, a passerine bird known for its powerful and beautiful song.