Yojimbo

Yojimbo (用心棒, "Bodyguard") is the ultimate achievement of both Shigenaga and his late grandmother, Nimhe Arma. It's an extremely powerful Shikigami-Ouji created over a period of three centuries, with some of the greatest and most divine materials available for purchase at the time. Although initially intended to be used for revenge against Nimhe enemies, it's current function appears to be to defend Shigenaga's household and the sorcerer himself from any enemies who'd try to kill him. Therefore it's name and function.

Like most combat-centric Shikigami, it's power is derived from noteworthy individuals whose spirits have been deconstructed and infused into it's body through magic. What separates Yojimbo from others of it's kind is however the quality of the souls and the infamy of the people which composes it. To get access to these souls, Shigenaga was required to make several deals, which included hiring skilled members of the Kidō Corps to get him in it's ritual of awakening, and also notifying Captain-Commander Shunsui Kyōraku of it's existence. Should Soul Society ever truly be threatened, Yojimbo is to serve as one of their trump cards, with such a promise made, Shigenaga was allowed to keep both his creation, and his life.

History
Yojimbo was the only Shikigami produced by Shigenaga Arma's late grandmother and mentor, Nimhe. It was intended to be her tool of revenge against those who had desecrated the name of her and her sister, to punish those who had taken her away. Such bitterness became hatred, and with that hatred came Yojimbo. Through her knowledge of the spirit arts, the old sorceress transformed herself into a variety of shapes to get close to the eleven greatest swordsmen of her time - each heading a dojo of their own and with many students that followed them blindly. Accustomed as they were to honorable combat, none of them suspected that they'd be brought low by fiendish magic and devilish trickery. That is, all except for one: was her name, unlike her peers, she would not be so easily defeated by sorcery and each attempt to curse her proved futile, for she would not reveal the secrets that Nimhe required.

Realizing she had to do without her, Nimhe stored the souls in what was to be Yojimbo's torso, a torso made of the scales of the Genbu itself. Considered impenetrable to magic, blade or fist. How Nimhe managed to get her hands on such a powerful artifact one could only guess, but her clan experienced a sudden crisis in their economy around that time. Over the course of several decades, she worked to construct Yojimbo's body, and although she could not afford to add the scales of Genbu to every part of it's main body, she went to great lengths to retrieve other materials of exceptional quality. Although the main pieces of Yojimbo's armor is composed of Genbu's scales, there's places where the scales of dragons have been used instead. These are it's weak points, for while the scale of dragons is incredibly resilent to elemental attacks, it's weaker to physical blows and is by no means impenetrable.

Before Nimhe could reach the last step of it's creation, the ten spirits gathered had to settle in within the span of fifthy years, and it was during these five decades that she was bested and ultimately killed at the hands of her own grandson. Shigenaga soon became aware of Yojimbo's unfinished state and existence, and brought it with him when he abandoned his clan. It's believed that Nimhe wished for her dear grandson to complete the construct in her place, perhaps realizing that in her old age she no longer had the strength to complete it's ritual of awakening. Where his grandmother had failed, he gave his all to succeed. Believing that the ten masters thus far locked away within it were insufficient, he set his sights on taking the soul of a powerful captain: when killed the former Captain of the 11th Division  the choice naturally fell on him.

For fifthy more years Yojimbo slumbered, empowering it further, and then only a mere few years ago, did something miracolous happen; Zaraki Kenpachi killed Yachiru Unohana in battle. Knowing well that such an immensely powerful soul would take a long time to corrode completely, Shigenaga used his wealth at the time, which was great even back then, to make a deal with the Kido Corps, allowing him to retrieve a portion of her spirit which was then infused into Yojimbo's sleeping form. Despite the construct having completedly integrated it's energy within it's constructed body, the sheer essence of Unohana, considered one of the greatest swordsmasters to have ever existed within the Soul Society was enough to delay it's awakening by a further ten years. Whereas it had at most required a single year to integrate the spirits of the other swordsmen, proving that her spirit alone had experienced, mastered and learned ten times as much as any one of them.

Now at last, the time for Yojimbo's true awakening draws near, and the creation of the greatest Shikigami-Ouji of this millenia is to be heralded, more than three-hundred years after it's original conception. To Shigenaga, Yojimbo is the final gift given from his grandmother, and one he does not intend to squander or use needlessly. To further empower his creation, the incredibly powerful Shikigami was granted a worthy weapon, a katana forged from what is said to be one of Ryujin's teeth. Such a blade would not break, even were it to be swung by someone such as Yojimbo.

Behind the Scenes
Yojimbo's name and appearance is a homage to the same character from Final Fantasy X. One of the authors favorite games. The author decided to keep the name because it perfectly embodied what he wanted from the character.