Imu is the Forest God
These are the reasons why I think Imu is the Forest God (and some additional theories regarding Imu's power)
Also in this lore63% confidenceThe One Piece is the return of the Ancient Kingdom!
Also in this lore65% confidenceTHE FAILED WORLD THEORY
Also in this lore87% confidenceLineage Factor is the sole power system of One Piece and Imu's real weapon
Also in this lore66% confidenceMythical Hito Hito no Mi: Model Zaza
Also in this lore75% confidenceThe Man Marked by Flames, Vander DeckenAlso in this lore75% confidenceThe Man Marked By Flames Is The Last God's Knight
Also in this lore58% confidenceJoy Boy didn't lose the Ancient Kingdom's greatest power. He gave it up.
Also in this lore80% confidenceMy Blackbeard theory: The Man of Two Worlds
Also in this lore80% confidenceCHURCHWARD'S "SACRED SYMBOLS OF MU" REVEALS THE GREAT KINGDOM'S NAME, THE "D" MYSTERY, AND THE END OF ONE PIECE
Also in this lore59% confidenceImu Is Nerona Imu, the Immortal Survivor of the First Twenty
Chapter referenceChapter 1138The Elbaf Mural
In Elbaf and Shandora, we've heard about four different gods in the One Piece world with legends that spread across different races. Their name may differ a bit, but I think they're still the same gods and I think one of the gods is Imu.
IMU IS THE FOREST GOD
Chapter referenceChapter 1044Nika's Drums of Liberation
Chapter referenceChapter 1182Zaza's Rhythm of the Rain Dance
Chapter referenceChapter 1179Imu's Choir of Domination
First of all, Imu has the same rhythm that both Luffy's Nika transformation and Qilingham's Zaza monster had. I believe this will be a common trait of the four gods. Just like how Nika's rubber-like power makes his heartbeat sounds like the Drums of Liberation and Zaza's storm power makes her dance sounds like rain (Dance Powder connection btw), Imu's powers put the devils that he control inside inanimate objects (oddly wooden things) and make them sing.
Chapter referenceChapter 1181Imu explains his power
Chapter referenceChapter 1179The name of Imu's power
Moreover, Imu also doesn't have a typical power. The power system in the One Piece world consists of willpower (Haki), dreams (Devil Fruit) and knowledge (Science). There are a few exceptions but they are very small and rare. Imu's power doesn't belong to any of those groups and it's instead called The Devil's Fruit. His power seems to revolve around controlling devils. Domi Reversi is him forcing devils into his enemies, the Covenant is him making contracts to allow his allies to borrow these devils' powers, and the Omen is Imu's personal army of devils. This is in line with what the Harley said the Forest God could do:
"The Forest God sent forth devils."
- The Harley, Chapter 1138 (Artur The Library of Ohara's translation)
ADDRESSING OTHER POSSIBILITIES
However, there are other theories for what Imu could be such as the Sea Devil. I think this is redundant because we know that the Forest God already has devil-themed power, so I think having another devil-themed mythical being would be too similar. Other than that, there are also theories that Chopper or Ryokugyu has the Forest God fruit, but that doesn't affect the theory at all because just like how Luffy has the Nika fruit, they can also still have the Forest God's fruit while Imu is the actual Forest God himself.
There are also people who think Imu would be better as the Earth or Sea god because of other parallels, but I think there are equally as much, if not more connections between Imu and the forest. First, Imu represents the Earth when compared with the Elders and the Ancient Weapons, but the Earth God actually translates to Land or Great Land instead of Earth. Forest, land and sea would all still be a viable option. Some people think Imu would be connected to the sea because his name spells out Umi (sea) when reversed in English, but there are other reasons for Imu's name like a reference to Buddha or King Louis XVI, so it is possible that it is a coincidence.
Chapter referenceChapter 653Shirahoshi talking about wanting to see a forest when she goes to the surface
Meanwhile, Imu lives in the Flower Chamber, right next to the Eve Tree and in the middle of an artificial oasis. Even his full introduction was done in Elbaf. Shirahoshi also said that she wants to see the forest when she got to the surface and the first place she goes to above the water is Imu's home.
ADDITIONAL THEORIES
Chapter referenceChapter 1189Sommers' sword is possessed and given powers
Chapter referenceChapter 291Trees of Shandora
Big Mom's soul power and Moria's shadow power are some of the foreshadowing for Imu's power and both of them are soul-related. Moria's shadow power is based on an Egyptian concept of souls and it's used to possess people like Imu's Domi Reversi. The relation to souls reminds me of the trees in Shandora that collect souls. If this power really is soul-related, I think Imu's 19 weapons are not the actual weapons of the 19 Monarchs but their souls instead. This would explain how these weapons possessed Sommers and Qilingham's weapons and even gave it powers as if they've eaten a devil fruit. This would be a good opportunity to finally explore that whole concept.
Chapter referenceChapter 0Simple sketch of the Red Dessert
Imu also has a lot of similarities with the character God Emperor Leto II from Dune, who is a huge inspiration to a lot of sci-fi emperors. Maybe I'll do another theory for that but the part I want to talk about is that Leto's power came from the sandworms that live in the desert, so when he saved the desert planet, Arrakis by making it full of life again, he made sure there is a little desert where he and his sandworms can live. I think Imu is doing the same thing too. He lived in the middle of the Red Desert and the whole world is flooding but Imu cannot leave his city of artificial oasis or even the Flower Chamber. That means these devils may depend on these plants to sustain themselves. These plants might tie back to that same plant from Shandora too.