The way you describe it reminds me of Irys from the Heisei Gamera series.
Basically there was this girl whose parents were killed as a result of collateral damage while Gamera was fighting Gyaos and she grew to hate him as a result.
She found a baby Irys as a child and raised him as her closest companion. He grew to immense size and power and became Gamera's nemesis. It was theorized that he was a Guardian of the North or some such.
Maybe that's Imu's origin and story. He was some demon or creature who was found and raised by Nefertari, who may have hated Joyboy for some misguided reason.
Hence his obsession with Vivi who reminds him of his only true friend maybe.
Interestingly both Irys and Imu have tendril based attacks, Imu grants power to others while Irys absorbs power through his tentacles.
Idk if any of this is possible tbh, but then again I do think Oda is a kaiju fan, he designed VA Red King after all (Not sure if that was a fan request).