Nightfall Menace

Nightfall Menace( Naitofōru menasu ​), also known as Colourless Shadow Commander, nicknamed "Nightingale"(ナイチンゲールちゃん; Naichingēru-chan) is the Sacred Gear of Ryuuji Kuwata and his partner as a member of the Zodiacs, sharing with him the position of the dragon(龍; Ryū) in DxD: Twice Critical.

An independent-type High-tier Sacred Gear which possesses human-level intelligence, it has the ability to change its appearance to any animals under the user's command.

Summary
First introducing herself as Ryuuji's partner among the zodiacs, Nightfall Menace using her alias Nightingale 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 Takumi's curiosity and interest once he felt she wasn't human.

It is later revealed that Nightingale is in fact an independent-type Sacred 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, making it clear that she wasn't in fact Ryuuji's real Parner.

Appearance
Being a Sacred 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 Ryuuji's possession, it takes the shape of a dark-haired, pale young woman of red eyes and wearing a school winter uniform to reflect on its User's "unusual interests".

Abilities
A high-tier, independent-type Sacred Gear, 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, however, is the ability to shapeshift and transform itself in 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 choosen animal(A.i. A lynx's speed, a hound's ferocity, etc.) while maintaining the same level of intelligence and memory. Both Ryuuji and Nightfall Menace can also share information with one another through a mental link between the two, and using his magical knowledge, he can channel elements into it for a more devastating effect.

Weaknesses
As limitless as the transformations seems, Nightfall Menace can only turn into living beings, meaning minerals, guns, constructions or even other forms are not a possiblity. Moreover, the transformations are limited to only one animal per turn, and in order to transform, the sacred gear must prepare itself in a cocoon-like sphere in order to take the desired shape, leaving vulnerable and defenseless in areas with a great number of enemies. Any damage taken by it will also reflect on the user, so any cut or hit against it might be proven fatal for both. The Sacred Gear can only act if summoned, and can only withdraw back into the shadows under the user's command. Since the shadow domain is its place for rest, it cannot communicate with the user with it, and unlike Canis Lykaon, it can't attack from the shadows either. The area the Sacred Gear can act away from the user is also limited to 10m, and it grows weaker with each passing distance between the two.

While Loyal to its master, Nightfall Menace's personality prioritizes its well-being and self-preservation above all else, meaning it'll listen to its user but might start questioning it and even refuse action once the situation has prove itself dangerous or even suicidal. While the Sacred Gear has its own stock of energy, meaning the user does not require a certain level of power in order to use it, it requires food and nutrients in order to properly work like any animal would, and if this requiriment is not met, the transformations will become weaker and slower, even limiting the amount of animals the Sacred Gear can morph into; moreover, it has an immense appetite, Ryuuji claiming a full-course meal for 50 people is only enough to satiate 1/10 of its hunger, and the more the user uses it and its abilities, the hungrier it gets.

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, devils, angels 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; Nekonekochenji: 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; Inuinuchenji: Burakkuhaundo): Nightfall Menace's canine transformaton, it takes the shape of a large and intimidating red-eyed hound with sharp teeth and razor-sharp dark fur, with and 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; Toritorishenji: 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 spionage 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; Uddouddochenji: 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 inherited abilities, such as the Bael's power of destruction. The defensive transformation, but it can't move or communicate.

Balance Breaker: Nocturne Menace
Nocturne Menace is Nightfall Menace's Balance Breaker. TBA.

Trivia

 * Images and appearance based on the character Oryou-san from the Fate/ series. The idea of a guardian spirit and its ability is also highly based on the concept of Stands from JoJo's Bizarre Adventure.
 * According to Ryuuji, Nightingale's chose her own nickname, and her favorite food seems to be cheap candy and junk food.
 * Nightfall Menace apparently can communicate with other sacred 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 passarine bird known for its powerful and beautiful song, keeping with the animal theme naming in DxD: TC.