Chainsaw Man's concept of Devils embodying fears is an intriguing one. As seen through 197 chapters of the series, the possibilities are nearly endless of what can and can't be. Taking it a step further, humans are also able to form contracts with these Devils - the ability to utilize their power for something in return, varying on a case-by-case basis.
Among the many holding Devil contracts, one of the most prominent and significant was Aki Hayakawa. Series deuteragonist Aki had contracts with the Curse Devil and the Fox Devil before later forging one with the Future Devil, who chose to reside in his left eye. Given his use of the Fox Devil, many wondered what happened to the Fox Devil after Aki's death.
Chainsaw Man: The Fox Devil after Aki's death, explained

Answering the question - what happened to the Fox Devil after Aki's death? - it continued to live. The story doesn't explicitly state what happened to it after Aki's demise, but the most logical guess is that it continued living in Kyoto. After all, Aki wasn't the only one it had contracted with, the Devil having forged contracts with Hirokazu Arai, Nomo, and several other Devil Hunters.
It was last seen during the International Assassins Arc, with Nakamura summoning its front paw to crush Santa Claus' puppets. Other than this arc, it has appeared in previous arcs like the Bat Devil Arc, Eternity Devil Arc, Katana Man Arc, and the Bomb Girl Arc. Through each of its summonings, it is learned that the Fox Devil tends to be picky.
This is with regards to the humans it contracts with, allowing only the attractive ones to summon its head, showcasing quite a superficial side. Moreover, it has no issues killing other Devils. But it does seem to dislike the taste of Hybrids, the examples being Katana Man and Reze. In both cases, once it tastes them, it decides to leave due to their "poor taste".

In other words, the Fox Devil doesn't seem to have a sense of loyalty towards anyone or anything. These instances showcase that Chainsaw Man's Fox Devil was quite willing to simply leave its contractors or outright break the contract if the situation appeared to be heading to an unwanted or undesirable outcome.
Such an attitude likely meant that Aki's demise reduced its number of contractors by one. Given that it felt no sense of loyalty, it didn't care much that Aki was gone. In truth, since Aki's demise, no mention of the Fox Devil has been made. A logical train of thought would dictate that it went on living in Kyoto, emerging when summoned by Arai and/or others when needed.
But this in no way states that the Fox Devil was weak. Standard Devil abilities aside, it possessed enhanced strength, durability, and regeneration. What made it fearsome was its ability to teleport to its contractor's location through a hand sign and a "Kon" chant. Afterwards, it would disappear in a puff of smoke. Effective against Devils, Hybrids could dodge Fox's paws and withstand its bite.
In Conclusion
Chainsaw Man’s Fox Devil continues to be an enigmatic presence in the series, more so following Aki Hayakawa’s death. While the story doesn't address its whereabouts, logical inferences point to its leading its life in Kyoto, maintaining contracts with other Devil Hunters like Arai and Nomo. Through the initial arcs of the story, the Fox Devil demonstrated a superficial and selective nature, aiding contractors based on physical appeal and abandoning them in a pickle.
Although it lacks a sense of loyalty, the Fox Devil is formidable, boasting enhanced strength, regeneration, and the ability to teleport to its contractor’s location instantly. Then again, its effectiveness is limited against Hybrids, as seen with Katana Man and Reze. Whatever the case, the Fox Devil’s absence after Aki’s demise aligns with its detached nature, implying that it simply moved on, possibly active yet uncommitted within the Chainsaw Man verse.
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