Kietasenshi

Ōe Shin’ichi was a samurai. Born in the samurai caste, his life and his duties had been decided from the start – long before the start, even, generations previously when his clan had come into existence and begun to take up positions in the imperial court in Kyoto. Though his clan had found renown as Confucian scholars, as poets, and as counsel to Emperors, Shin’ichi himself was a far less important member of the clan, in terms of nobility, and so saw only generic roles in court.
Shin’ichi grew to develop a very, very decided view of that portion of the samurai lifestyle. That is to say, he came to absolutely despise it. Sitting still all day, listening to prattling on, maintaining face, trying not to get in too much trouble for giving the cold shoulder to socialites who he suspected were constantly lying their asses off… he hated it. Tea ceremony? He hated that too. Flower arrangement? He hated flower arrangement, too. Poetry? Oh, you better believe Shin’ichi hated poetry.
Instead, Shin’ichi found his joy in the more martial duties of a samurai. To say that Shin’ichi thrived on the battlefield would be a profound understatement. His analytical tendencies, to take in the world around him through keen sensory inputs and responses and rapidly break it into chances and risks had served to fuel his cynicism and boredom with court life, especially with a lack of enough emotional understanding to excel in it… but these tendencies let him flow around the field of combat with a natural grace, especially combined with a natural intuition that often allowed him snap decisions in the midst of brutal, frenetic combat. He loved the physical stimulus of combat, the instant feedback of parrying a blow or stabbing through a foe. He loved not having to restrain himself behind some silly stoicism.
Shin’ichi ended up finally finding some renown on the battlefield after two and a half decades of his life mostly in court – not enough to become, say, a name recognized throughout Japan, but enough to gain a little fame within his clan. This was enough to excuse increasing court slip-ups (as Shin’ichi stopped giving a damn) and enough to secure him better and more important positions in battle. Of course, as the saying goes, “he who lives by the sword dies by the sword”, and it wasn’t long before his rise to fame was interrupted by an arrow through the head.
Shin’ichi didn’t really mind dying this way, actually – he had figured that a death in combat was pretty much the best death that he could ask for. The problem was what came after. Shin’ichi had not gone through twenty-six years mostly composed of mindnumbing restrictions, courts, and superiors who viewed him as definitionally expendable to go wait around in some dank underground river or some shit like that. No, that he was absolutely not willing to accept, not when he’d finally been coming so far, and so Shin’ichi simply… refused to pass into the afterlife.
And, actually, it worked a lot better for him that he had ever anticipated it would. He totally got away with it, and now living life as a youkai, the samurai proceeded to indulge his preferred lifestyle. He would wander throughout Japan, finding battlefields and skirmishes and appearing in them – while he certainly did not have the powers of a god, his presence was oft a terrifying one, with the youkai quite proficient in combat and not afraid of death in the way a living human would be. Then, by the time the battle ended, he would be gone. The youkai became known as Kietasenshi, the Disappeared Warrior: his propensity for combat was as great as his sudden entrance and exit from battle.
For many years, Kietasenshi enjoyed this battlefield-wandering life, but sometimes it wasn’t enough. Every decade or so, he would feel the need to challenge something greater – and he found a youkai to fight. He was *usually* good at choosing his targets and battling against fellow lesser youkai usually saw him win. Eventually, though, Kietasenshi desired an even greater challenge, and traveled to a forest where he had heard rumors of an onmyoji living within a forest – this onmyoji, of course being Yoriie. He challenged Yoriie to a battle, to the onmyoji’s probably annoyance… and though he put up a fight for a while he did, in fact, get his ass kicked, and because having a youkai running around fighting onmyoji was not exactly going to be good for the order of the spirit world, he was promptly sealed under Yoriie’s service.
Kietasenshi was fine with this at first, as it wasn’t like Yoriie was going to make him start writing poetry… but Kietasenshi eventually realized that in an out-of-the-way forest like this Yoriie wasn’t going to make him do much of ANYTHING, and so from then on he became something of a pain in the ass with Yoriie, starting fights and spars constantly with his fellow youkai and always itching to take the combat solution to any problems that showed up… probably to the point of frequently having to be forcibly restrained by his new master.