I tend to doubt that Imu will turn out to be the literal Devil. Lucifer and/or Satan, themsevlves.
...But, I could see Imu ending up having some parallels to either Lilith and/or Naamah from the Bible, or maybe Echidna from Greek Myth.
https://en.wikipedia.org/wiki/Lilith
https://jwa.org/encyclopedia/article/lilith
https://www.britannica.com/topic/Lilith-Jewish-folklore
https://en.wikipedia.org/wiki/Naamah_(demon)
https://blog.nli.org.il/en/naamah/
https://en.wikipedia.org/wiki/Echidna_(mythology)
https://www.theoi.com/Ther/DrakainaEkhidna1.html
I doubt any source of inspiration will be a clear 1 to 1 with Imu. But, I could see bits and pieces from all these sources ending up influencing Imu's backstory.
Personally, I'm betting that Imu's human form will end up looking like a female child. From everything we've seen, Imu seems to be very short, like a child. And, personally, I think Imu will turn out to be not as much of a villain as some people expect. Imu's in the wrong, but probably for understandable and ultimately sympathetic reasons. And, if that's the case, I feel like it'd be easier to forgive a female character than a male one, for the audience. Especially if they also look like a child. That kind of character design would make them a lot more sympathetic looking, making it easier for the audience to forgive their MANY crimes. I don't have any "proof" for this. I just think it'd make for a easier redemption arc, if Oda tries to make Imu sympathetic.
And if that's the case, the "Mother of All Monsters" archetype kind of fits. Especially if Imu also turns out to be the Forest God mentioned in the Second World from the Chapter 1138 Mural. The one who "sent forth devils". Meaning either creating "Yokai Monsters" like the Gorosei, or just being the source of all Devil Fruits, in general. Imu actually makes a lot of sense to have the Forest God Fruit, in my opinion. They live on top of the Sunlight Tree Eve on the Red Line. They spend most of their time in the Flower Garden. The Forest God fits Imu. It's even possible that we're seeing all these tree-related diseases in the One Piece World, because Imu themselves have some sort of disease, and the Forest God powers are effecting trees all around the World. Maybe.
Whatever the case, I'm also expected Imu to have undergone the Immortality Surgery. If only because...SOMEONE really has to have had that done to them, in order for people to know about it. And Imu is the most likely candidate we know about. It's entirely possible that Imu predates the Void Century. Maybe even going as far back as the First World, or Nika's time.
As for what Imu actually is...I have two different theories that I like.
One is that Imu is the last "pure" Human left on this World. One Piece is focusing more and more on "hybrid races". And it's entirely possible that EVERY Human on the planet has some level of mixed blood. Most people might have a bit of Giant or Fishman blood in them, even if they look mostly Human. There's even some speculation now that Usopp could be a half-Tontatta, half-Giant Zushindodo, even though we've assumed he's just a Human with a long nose for the past nearly 30 years. It's possible that the reason why everyone in this World looks so cartoony is because they're all hybrid races, on some level. Imu, therefore, could be a "realistic" Human. And I just think that'd be a way for Imu's appearance to surprise people. Imagine Oda drawing in a completely different, hyper-realistic art-style JUST for Imu alone. A Human in a world of Toons, like something out of Roger Rabbit. I just think it'd be a interesting way to surprise people. It'd also really drive the theme of "intolerance" home, if Oda wanted to go in that direction.
The other idea is that Imu could be the last living Dragon left in the World. If you know me, you know I believe that the Dragons are indeed coming, and they WILL be amazing. My pet theory is that the Dragon races were the original native peoples of this World, and that invading Humans from outer space stole this planet from them, colonizing it for themselves. As kind of a metaphor for the real-life Ainu people, who were also almost erased from history, and replaced with depictions of demonic Oni. And if Imu has the Forest God Fruit, this actually would make some sense. Imu's "human form" is actually their Zoan "animal" form, since the God Fruits are all actually Mythical Human-Human Fruits. With Imu's "Dragon" form actually being their original appearance. This would also tie in with the various hybrid races. Because, I believe most of these hybrid races come from mixed breeding with the original Dragon Races. The Ancient Giant's horns and eyes, are VERY similar to the horns and eyes of the Dragon that shows up in Ryuma's "Monsters" one shot. Imu's "demonic" attributes like the wings and horns and tail, are just as "draconic" as they are "demonic". Imu would just be a self-hating Dragon who decided to save the Humans, and turned against their own kind.
https://www.abc.net.au/news/2023-10-03/japans-indigenous-ainu-people-fight-for-recognition/102903914
https://raestea.medium.com/japans-l...to-the-ainu-people-olympics-2020-ab7410523ab3
https://www.nippon.com/en/japan-topics/g02276/
As a variant of the second idea, Imu could also basically be the "First Hybrid" between the Humans and the Dragons. In chapter 722, we see Giolla make her "Dying Art". A piece of living art that shows a Dragon-like creature, and a Robot-like creature trying to keep a door closed, with someone in a crown cowering in the foreground. This could be a hint that Imu is literally "stuck between" both groups. Part of both, and neither at the same time. Wanted by neither group for being a freak. I feel like this could also be a possibility. And perhaps that's the reason why Imu still hides themselves away to this day. They know they'll be seen as a freak by everyone in the World.
I'm not sure which idea I like better. I feel like they'd both make for interesting reveals that could take most people by surprise.
But, yeah. Most likely not the literal devil. But, Imu most likely is someone who has been "demonized", probably somewhat unjustly, by those around them. I''m calling it now. We're all going to be crying over Imu's backstory, whenever we get around to hearing about it. "Sympathy for the Devil" is a phrase for a reason.
