In his brief appearance, Isshiki reveals himself to be a cruel, ruthless and pragmatic being. It's unclear how much he influences Jigen, but he shows him no care or respect at all, curses at his limits and degrades him as a worthless vessel. Having deemed Jigen's body as weak, Isshiki strongly desires in possessing Kawaki and turning him into his new body. When in control, Isshiki is shown to be noticeably more arrogant and battle-hungry than Jigen, as he would mock and taunt Naruto and Sasuke for their poor efforts, but is not above admitting their strategical attempt was a good one and even stated he felt entertained by the fight and expressing bizarre gratitude before decisively defeating them. Isshiki demonstrates a level of lenience when he had Naruto Uzumaki sealed away for further use in the future than to kill him, despite the latter's insults, although it may be just a calculated decision as he would likely see Kawaki's attachment to the Hokage as being both an annoyance and something he could use to force him to cooperate. Along with that, he openly noted that Sasuke was welcome to escape by himself if he could leave Naruto behind.
Isshiki's power is so great that unleashing the full extent of his power via the Kama he gave Jigen was enough to overwhelm his body with his immense chakra and cause it to break down. Regardless, he was still able to decisively defeat both Naruto and Sasuke at once.
As Jigen's powers likely came from his Kama, Isshiki should be capable of levitation, chakra absorption, manipulating the size of objects and conjuring black rods capable of draining chakra. It is unknown, however, if Jigen's use of Genjutsu to communicate with all the Inners of Kara or his extremely powerful sealing abilities, strong enough to seal away Naruto in a coffin, came from Isshiki, but it's extremely likely the latter does come from him.
Not much is known about Isshiki's past, but he was paired with Kaguya Ōtsutsuki and they arrived on Earth. Kaguya, some point after eating the Chakra Fruit, apparently seeing Isshiki as too much of a threat, caught him off-guard and mortally wounded him. On the verge of death, Isshiki was forced to enter Jigen's body via his ability to shrink himself, entering Jigen's body via his ear and taking control of it and branding him with his Kama. However, Jigen was too weak and would die even if he were to resurrect and as such, he founded Kara and molded Kawaki into becoming his future vessel.
When Kawaki goes rogue, Jigen tasks Kara with going after and retrieving him. At one point, Jigen express interest when he hears that Boruto has a kama seal from Delta and thinks of Momoshiki and his plans. Jigen visits the Ten-Tails in an alternate space-time and tells it that he is not ready to be eaten by it. Jigen manages to absorb a lot of chakra taking on the form of Isshiki briefly. He later visits the Hidden Leaf Village where he tried to retrieve Kawaki, but was unsuccessful and instead encounters and fights Naruto, the Seventh Hokage. Jigen warps Naruto to another dimension and fights both him and the recently arrived Sasuke. Once he was driven into a corner, Jigen activated his full Kama and overpowered them, but since the latter expend so much chakra, it awakens Isshiki's personality. Going forth, Isshiki realized how dire his condition is as Jigen's body can't keep pace with his full power for long and he really needs Kawaki. Despite that, Isshiki in possession of Jigen incapacitates Naruto and Sasuke with his powers. Isshiki, knowing that killing Naruto would take too much Chakra, prepares to seal Naruto and kill Sasuke, but the latter left as Naruto tells him to go. Isshiki proceeds to seal away Naruto and the Nine-Tails as he reveals he was never interested in any of them but only targeted Boruto, though he likely wants to use Naruto as a hostage for Kawaki to cooperate and threatens him with death if they meet again. As Isshiki shrinks the coffin, Jigen's body is finally at his limit and collapses, even shedding a tear of pain, and Isshiki curses at Jigen's limits as he is forced to return to Kara's base before giving back control for Jigen to recover his powers.
- It is heavily implied that Isshiki eventually succeeds in turning Kawaki into his new body, as the latter would later destroy the Hidden Leaf Village and (apparently) kill Naruto before trying to do the same to Boruto.
- Since Isshiki is in the same clan as Hagoromo Ōtsutsuki, the ancestor of Naruto Uzumaki, he is one of Hagoromo's cousins and is therefore related to Naruto.