Isshiki Ōtsutsuki

is a member of the Ōtsutsuki clan's main family, currently using Jigen as his vessel.

Background
Isshiki came to Earth alongside Kaguya Ōtsutsuki, but she betrayed Isshiki and left him at the brink of death, the lower half of his body destroyed. While in a weakened state, Isshiki came across Jigen, and secretly shrunk in size and entered Jigen's head through his ear, and began surviving off him like a parasite, eventually taking control of his brain and marking him with the Kāma. Because Jigen's body wasn't powerful enough to sustain him should he be fully reborn, Isshiki opted to obtain a new vessel, experimenting until he was able to pass his Kāma to Kawaki.

Personality
Isshiki is arrogant, pragmatic, and ruthless. Though it is unclear exactly where Jigen's personality ends and Isshiki's begins, Isshiki is unconcerned with the damage his overwhelming power inflicts on Jigen's body, mocking his tears of pain and declaring him to be a worthless and defective vessel, and he is also shown to be able to enjoy battles and outright mock enemies, as he repeatedly insults Naruto and Sasuke upon overpowering them decisively and relishes in their expressions of awe and fear of his power, even going as far as to say he enjoyed the battle and thanking them for the fun. Despite his ruthlessness and arrogance, Isshiki is shown to be able to remain calm even when faced with insults, ignoring Naruto's mockeries and sealing him instead of killing him as he doesn't want to expend the chakra needed to kill Naruto due to Kurama. He desires Kawaki, who has been groomed as his next vessel, and is interested in Boruto Uzumaki, who bears Momoshiki's Kāma.

Appearance
Isshiki dressed in an elegant, slim cut robe with an oversized collar and extremely long sleeves, over a pair of baggy black pants tied at the ankles. The robe was embroidered with vertical columns comprised of eight magatama designs on the sleeves, and a similar pattern on the back, with the second magatama replaced with a black emblem resembling an eye. He bore some telltale features of the Ōtsutsuki clan, including a zig-zagging part in his hair and a long, curving horn, which grew from his left temple and wrapped around the back of his skull before jutting outward behind his right temple, resembling a crown.

Abilities
Isshiki's power is so vast that Jigen's body has difficulty containing it, preventing him from using the full extent of it for long. Despite his Kāma enhanced vessel being at its limit, he was still capable of simultaneously defeating the two strongest shinobi in history.

Physical and Chakra Prowess
While possessing Jigen upon him using his enhanced Kāma, Isshiki displays such a tremendous level of physical strength and speed that he was able to effortlessly dodge Sasuke's Complete Body Susanoo and Naruto's Tailed Beast Mode attacks and plunge them out with a single attack as well as pummel them severely with just a few punches and kicks. He is also resilient enough to be completely unfazed by being burned by Amaterasu, which could harm even Tailed Beasts, proceeding to calmly absorb it.

His level of chakra is so vast and powerful that his full power is capable of overwhelming Jigen's body with the sheer intensity of it.

Ninjutsu
Isshiki has access to a range of techniques including manipulating the size of matter, teleportation across dimensions, chakra absorption, and levitation, which he bestows upon Jigen with his Kāma. It appears when he is directly in control of Jigen when he uses the full power of his Kāma, he is capable of using these techniques with noticeably greater potency, as not only could he instantly summon large black rods to impale Naruto and Sasuke, but he was even able to manifest rods directly from his body to block Sasuke's sword strike. He was also able to effortlessly absorb Sasuke's Amaterasu.

Kawaki Arc
While attempting to capture Kawaki from Naruto Uzumaki's household, Jigen transported Naruto to another dimension with the intent of abandoning him there to die, but was attacked by Sasuke Uchiha. During the ensuing battle Jigen was driven back and resorted to unleashing the full power of his Kāma, manifesting Isshiki's personality. As he quickly overpowered Naruto and Sasuke, pummelling them into submission, Jigen's body began to break down, which frustrated Isshiki as he disabled Naruto and Sasuke with giant black rods and reaffirmed his dire need to possess Kawaki. Revealing the crater acting as their battlefield was actually a gigantic coffin, Isshiki intended to kill Sasuke as he could escape any time and seal Naruto and Kurama within it, as he couldn't exert the power needed to kill Naruto due to Jigen being weakened. Sasuke escaped with Naruto's help and Isshiki brushed off Naruto's insults and revealed he was never interested in him as Kurama noted and that he was after Boruto Uzumaki for his Kāma, promising to kill Naruto if he escaped as Isshiki successfully sealed Naruto within the coffin, which he shrank to a normal size. Before teleporting away, Isshiki cursed Jigen's weakness as a vessel.

Trivia

 * Like other male members of the Ōtsutsuki clan, Isshiki was named after a Japanese folk hero whose name includes "Tarō", in this case Issun Kotarō, a hero from Japanese folklore who was only an inch tall, popularly known as Issun-bōshi. As a boy he went on an adventure to rescue a girl from an ogre, triumphing after being swallowed by it and using his sword — a needle — to attack it from within. He then claimed the ogre's magic hammer, Uchide no kozuchi, and used it to transform into a full-sized adult.