He's there just because he's the antagonist. That's the problem though. "Being the antagonist" is the only motivation behind his character so far and that's simply not great writing.
It’s even worse than that, because Kaido so far is established to have two conflicting motivations: to be the Pirate King (I assume this is the ultimate goal of his desire to start the world’s biggest war), and to die.
These motivations are in conflict and it does not make sense for him to have both. I guess you could say Kaido is an adrenaline junky at heart, and so what he really craves is just raw excitement? But if that’s the case, why does Oda draw him screaming in terror every time something happens to him? Literally this guy screamed in terror with the scabbards were giving him minor cuts and pushing him out of a window.
The guy who jumped 10,000 meters off a sky island, screamed in terror when the scabbards pushed him out of a window.
Akainu for reference, gritted his teeth through two earth quakes to the head and stomach, but Kaido is screaming his brains out when the scabbards give him paper cuts? Doflamingo laughed as Gear 2 punched him in the chest, but Kaido screams in pain from Killer?
If Kaido were a true adrenaline junky, he would be laughing off such minuscule injuries. I think the real problem is that Oda thinks that he is artistically incapable of actually showing Big Mom and Kaido getting injured, and so he has to draw both characters screaming in pain after taking small injuries so readers actually know they are being hurt?? Even if that does result in Big Mom and Kaido looking like pansies with no endurance whatsoever?
The sad thing is that this could’ve easily been avoided if Oda wouldn’t have made the
incredibly stupid decision to make Kaido and Big Mom invincible freaks of nature. They should’ve just been relatively normal flesh and blood characters like anybody else in OP, now it’s a mystery how much damage they’re actually taking because on-panel it looks like they’re only taking minor wounds, but they’re screaming their heads off as if they’re having limbs blown off.
At the end of the day, this makes both characters look like little Bitches with zero pain tolerance, and it’s impossible to take them seriously as villains. I doubt Oda intended to portray these clowns this way, I think he wants us to perceive the Yonko as being legitimate threats, but like...imagine if Doflamingo was screaming in pain when Baby 5 shot him in the face with a bazooka. All of Doflamingo’s legendary charisma would’ve been flushed straight down the toilet instantly.
Well, welcome to Big Job and Crydo.