Going through Charlotte Linlin’s history something that struck out to me was the giant warrior captain Jorl’s initial belief that Charlotte wasn’t a child but instead an evil spirit in the form of a child. There’s actually a fair amount of commentary in one piece about evil spirits (Zoro’s cursed blade, Hakuba, etc.) but Jorl’s comment is striking in that beyond rumored suspicions, he seems to be suggesting that this a known phenomenon among giants: evil spirits causing havoc in the real world. It’s highly suspected that Kaido has some ties to the giants given how unreasonable large he is and I wonder if this is the ultimate connection Oda has planned: Kaido is a giant possessed by a yurei / onryo devil fruit user.
Kaido’s subordinates and his obsession with chaos: While many have looked as the four horsemen from Judeo-Christian mythos as being a template for Kaido’s commanders, there’s a counter explanation in Japanese mythos: Onryo, vengeful spirits obsessed with causing destruction in the real world. One such way is through natural disasters (earth, fires, droughts, etc from Wikipedia) which lines up perfectly with the revealed commander Jack, The Drought and potentially the others. On top of that, let’s look at Kaido’s philosophical interests — causing chaos for the sake of chaos. Such is the behavior of an Onryo that ultimately just wants to see as much destruction and disarray as possible.
Kaido’s hobby of suicide: So far, it seems most fans have taken this as merely a testament of strength; Kaido is so powerful the only way he can entertain himself is by trying to kill himself. But couldn’t this also be explained by Kaido really, truly wanting to die — to be rid of the thing/person possessing him. The fact that weapons and chains break when touching him could have less to do with his own constitution but instead the actions of his possessor who is using his / her power to shield the real Kaido’s body.
Precedence of Giant Possession in One Piece: Of course, Kaido wouldn’t try to kill himself if the being possessing him was in full control, which is why I don’t think it totally defines what he’ll do. If anything, there’s a precedent in one piece that explains this well in Moriah’s use of Orz’s corpse. Moriah wanted the raw power of a giant but lacked the means to control such a strong form (which is why he had to rely on Luffy’s shadow.) An extremely physically strong Kaido might somewhat be able to resist his possessor, hence the dissonance and erratic behavior.
There’s definitely some secret to Kaido’s strength: We are made to believe he is indestructible and uncompromising and yet Shanks successfully prevents him from venturing to marineford. Doesn’t prove he’s possessed by a demon but does indicate that things we’ve been told about Kaido don’t so far match up!
His interest in zoans: Onryo are misanthropes obsessed with tormenting people. Given their fierce dislike of humans, wouldn’t one want the least human crew possible: a group of wild beasts more invested in chaos than any sort of civil order? Similarly, Kaido is a giant turned beast as much of his self-control has been stripped of him to be the plaything of the person possessing him.
Theory by batoftherat