EK
Demons
Making her first appearance by destroying and consuming the blood of an entire village, Mika Akashiya quickly establishes herself as an adversary to Diamante Rose Uzumaki early on. The Curse Mark Orochimaru implemented on Diamante is weak, causing Mika to be able to come out at will.
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Mika was a vampire, in love with Kaname Uchiha, Sasuke's temporary Curse Mark Demon that ends up being killed by Itachi later on in the series. Being a vampire, she really does not care about human trivial things and agreed to be tested on by Orochimaru in her search for power. Orochimaru ends up deeming Mika too powerful, so he seals her essence within his Curse Mark, keeping her within his body for protection. He divided her power into seven phases: Snow Phase (pictured), Devils Phase, Goddess Phase, Purple Diamond Phase, Hells Phase, Death Phase, and Inferno Phase. Each phase multiplies Mika's power by 10; however, at the expense of her vessel. If she were to go into Death Phase, it would mean that at transforming back to the host's body, both she and the host will die.
At giving Diamante the Curse Mark, Orochimaru planted Mika inside of Diamante. As mentioned previously, Mika's first appearance was characterized by destroying and consuming an entire village, causing Diamante to be afraid and seek to master Sage Mode in search of a way to control this demon.
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After Sage Mode training, Diamante sets out to find the Uzushigakure Ruins in hopes of learning Fuuinjutsu and sealing Mika in a way that would benefit Diamante. After traveling and finding the ruins, Diamante teaches herself all there is to know about Fuuinjutsu using the scrolls she finds. There, she creates a seal that allows her to use Mika's power in her favor.
Kira is a Juubi from an alternative dimension that one day literally got too bored and broke away from her confinement. She traveled to the Shinobi World and found a suitable host that would not die from her immense power. Sadly, all she found was an infant baby girl. Undeterred, she let her essence go into the body of the said infant, waiting for the correct moment to make her presence known. She spent her time resting, until the day Diamante sealed Mika and traveled to her Unconsiousness for the first time. There, Kira was awoken by Mika's and Diamante's arguing, making her presence known for the first time.
Despite being the most powerful of the Juubi, Kira's personality is easy-going and lazy. She is a quite perverted fox, always somehow turning the conversation to sex. Kira likes to keep to herself despite her immense power, and only makes herself known when Diamante is on the brink of death or becomes severely wounded.
Kekegenkais
The Special Rinnegan is just as the name hints. It is a "special form"​ of the Rinnegan and it is much more powerful than the Rinnegan. It is said that what the Sage of Six Paths unlocked was the Special Rinnegan and was mistaken for the Rinnegan.
Shakuran
The Shakuran is known as the "Ultimate Kekegenkai" this of course, hints at the fact that there is nothing as strong or stronger than the Shakuran. The Shakuran has the powers of every and all visual kekegenkais, only it is a thousand times stronger and doesn't cause blindness like the Sharingan. For this reason, the Shakuran is the Ultimate Kekegenkai.
