The recently-appointed Karas of Shinjuku, tasked with preserving the balance between the worlds of humanity and the youkai by the Will Of The City... and also with taking down his renegade predecessor, Eko. A quiet, kindly young man who has acquired the nickname of 'Doctor Karas' due to the medical help and counselling he offers to sick youkai when he's off-duty. Used to be a hitman for the Yakuza, until an attempt to escape his life of crime led to him being fatally shot by his former comrades. The Will saw its opportunity, and gave him a job offer he couldn't refuse. One resurrection later, he was putting his considerable people-slicing skills to use as a preserver of the city and all-around hero.
Disability Superpower: He has CIP; Congenital Insensitivity to Pain. This explains his Determinator status as he literally can't feel any of the pain that normally leave a human a screaming mass of blood and gore.
Good Thing You Can Heal: It's lucky that Karas are naturally tough and fast-healing, because Otoha is downright awful at taking care of his body. After he gets Brought Down to Normal, this habit gets him into some pretty serious trouble.
Katanas Are Just Better: Even before he received the mighty Absurdly Sharp Blade that is the badge of the Shinjuku Karas, Otoha was quite capable of slicing his way through through entire gangs of gun-toting yakuza with a regular katana.
Ninja: Most Karas have some elements of this, but Otoha takes it all the way.
Parental Incest: His father is also his brother. It's implied that this is the reason for the whole CIP business.
The series's main villain, the original Karas of Shinjuku. Grew disillusioned with the city's human population over the centuries, thanks to their perceived corruption and abandonment of the old ways at the expense of the youkai. Eventually decided to go completely rogue, imprisoning his Yurine and launching an assault on the city with his hand-picked band of mechanical monstrosities to reclaim it in the name of the youkai, even if he had to enslave every human in Shinjuku to do it.
Knight Templar: Seeks to preserve the youkai by any means necessary, up to and including forcing them all to become vampiric cyborg abominations and turning the humans into little more than living larders for them. Doesn't see the slightest problem in this.
Light Is Not Good: He's a thoroughly nasty piece of work, and in contrast to the dark, sombre Otoha, he's almost always suffused in light and bright colours.
Oedipus Complex: Sees the Yurine as his mother and the Will Of The City as his father, and seeks to supplant the latter. Do the maths.
Who Wants to Live Forever?: It was the hundreds of years of watching the world he was assigned to protect slowly decay that sent him over the edge, and he firmly believes that the same will happen to Otoha in time.
A Karas from another town who popped over to see what was going on in Shinjuku. Prohibited from interfering with another city's business, but eventually cajoles her Yurine into letting her help Otoha out.
Arrogant Kung-Fu Guy: A mild, gender-flipped example. Mostly wants to intervene in Shinjuku in order to show off her superior abilities, but doesn't cause too much friction since by that point, Otoha needs all the help he can get.
Big Damn Heroes: Pretty much every time she's authorised to go Karas, she ends up saving Otoha's hide. Eventually, he returns the favour.
The Worf Effect: Set up as a more capable, experienced Karas than Otoha, but ends up getting ignominiously defeated and incapacitated by Eko in order to demonstrate his unstoppability. To be perfectly fair, though, our hero wasn't doing much better until she stepped in, and she bought enough time for him to come up with a Plan B.
A renegade Mikura who fled Shinjuku some time ago, Nue has returned to the city to settle old scores. Masquerading as a laid-back, homeless drifter who lives on the Shinjuku public transport system, he hunts down his former comrades whilst struggling against the bloodlust that threatens to consume him.
Monster Misogyny: Fine with eating just about anyone, but most of his victims seem to be women.
Necromancer: Capable of summoning the spirits of his victims to aid him in battle.
Eko's personal servant, Wanyudou is a calm, dapper gentleman in his human form, and a gigantic, wheeled abomination in his Mikura form. He feeds from motorists in Shinjuku's road tunnels, distracting them with his alternate form of a phantasmic red car before ambushing them from behind.
Transforming Mecha: Unique in that his Mikura form has a vehicle mode - two, if one counts the phantom car he uses as bait.
The sole female Mikura, Tsuchigumo is a quiet, beautiful woman half the time, and a giant, loathsome mechanical spider for the other half. She is loyal, efficient, and frequently assigned by Eko to the missions he deems most crucial to his cause.
Unlimited Wardrobe: Goes through an astonishing number of outfits during the course of the show, especially when one considers her relatively limited screentime.
The Worm That Walks: Her body is comprised of lots and lots of little spiders, which she can send out to do unutterably horrible things to her victims.
Perhaps the strangest of the Mikura, Kamaitachi is a whirling, bladed berserker with a grating, mechanical voice and no apparent human form. Given his high visibility, he rarely ventures outside Eko's headquarters except on the few occasions when the centuries-old Karas doesn't care about being subtle and just wants something dead quickly.
Monster Clown: He strongly resembles one in appearance, at least.
Razor Wind: Given who he was pre-Mikurafication, it should come as no surprise that he's capable of doing this.
The Unintelligible: His speech is so heavily filtered and modulated that he very nearly becomes this. Dub watchers have it even worse, since he's still subtitled in Japanese.
One of the most brutal and sadistic Mikura, Ushi-Oni takes the form of a hulking, gear-toothed bull/spider hybrid. His human form is decidedly more notable, though - he's the Shinjuku police commissioner, Narumi and Sagisaka's boss, who spent years as Eko's man on the inside in the city's local government in exchange for getting the lion's share of the food once his master's plan came to fruition.
Chekhov's Gun: In his human form he wears a ring like the other Mikura and it is clearly shown in the first episode.
I'm a Humanitarian: All Mikura feed on human blood to survive, yes, but Ushi-Oni is particularly notable for simply crunching his victims up and swallowing them rather than using some more exotic method to drain them.