WARNING: The following contains spoilers for Boruto: Naruto Next Generations Chapter #54, “Bro,” by Masashi Kishimoto, Mikio Ikemoto, Mari Morimoto and Snir Aharon, now available in English from Viz Media.
When Kawaki slammed the door on Isshiki Ōtsutsuki's resurrection in Chapter #53 -- killing him by stomping him to dust -- fans of the Boruto manga were utterly relieved as it meant Kawaki wouldn't be a vessel anymore. No longer was the boy going to be part of some sinister plan where he'd be turned into the real Isshiki after Isshiki (in Jigen's body) was fed to the God Tree. Instead, it meant he could finally return to Konoha and live a normal life.
Well, Kawaki can't count his lucky stars just yet because as Boruto Chapter #54 has now revealed, he is still very much a pawn in Momoshiki's despicable endgame to conquer the planet.
Momoshiki was waiting for a window to possess Boruto and the war against Isshiki provided just that. As Isshiki fell, Boruto's chakra levels ran low, allowing Momoshiki to take control of the teen's body and stab Sasuke's Rinnegan eye. Here, we see the aftermath as Momoshiki goes after Kawaki, all while trying to finish off Sasuke. To make it worse, Naruto's out cold which means the heroes are at a severe disadvantage, especially as Momoshiki is using a lot of Ōtsutsuki power against them.
Kawaki and Sasuke are very curious as to why Momoshiki needs Kawaki's body and the Karma mark, as they think it's no longer a proper Ōtsutsuki vessel. Well, that's not the case as Momoshiki reveals when Isshiki's DNA started spreading through Kawaki, the process was about 80 percent completed. So, while Isshiki's consciousness is gone, Kawaki is very much an Ōtsutsuki vessel, meaning he can be used as food for the God Tree instead of Jigen/Isshiki.
As a result, Momoshiki, in Boruto's body, can then eat the chakra fruit produced and achieve ultimate power and immortality. Granted, the fruit won't be as big but it'll more than suffice. It certainly beats sacrificing humans who wouldn't have much chakra to juice the fruit. This leaves Kawaki pissed as once more he's robbed of his agency. He hates how he's being treated like an object, which is why he wants to free Boruto's mind too. Exorcising Momoshiki will give his adoptive brother his freedom back so Kawaki, whether or not he has alien DNA in him, wants a fight to the death.
As of Boruto Chapter #54, it's not going to be easy for Momoshiki, however, because apart from having to kidnap Kawaki as his new vessel, he also needs the Ten-Tails. He's not sure where Isshiki hid the beast and seeing as Code's protecting it for Kara, it'll be under heavy protection in its secret dimension. But for now, the first piece of the puzzle is Kawaki because if he can attain him, it'd mean Momoshiki has more than likely taken out his main opposition in Konoha's ninja.