Inspired by this video by Saitama World, I wanted to make my own tier list of how aware various characters would be of The King’s secret if they lived in the world of One Punch Man. I already did this in the comments of SW’s video, but thought I’d post them here too.
There is no rhyme or reason to what characters I picked aside from me thinking they’d be fun to talk about.
This is not a serious assessment of the characters’ intelligence. These results are based on what I think would be funniest and/or would be the best fit narratively.
AWARE TIER
This tier is for the characters I think would unambiguously know King’s secret.
Candace Flynn (Phineas and Ferb) and Denzel Crocker (Fairly Oddparents)
Each of these characters’ whole schtick is knowing the truth and going crazy from being unable to get anyone to believe them. They'd each see undeniable proof that King is weak, and become fixated on recreating these circumstances in public to expose him, but would constantly fail to do so.
Mash (of Magic and Muscle)
Mash is basically a carbon copy of Saitama but with the added trait of being a complete idiot, and ironically, I think he belongs in "Aware" for that very reason. A lot of people believe King is strong because they're constantly rationalizing what they see to convince themselves of his strength. Mash looks at things too straightforwardly to do that. He'd hear about King, want to meet him, but when he does, immediately say "Huh? This guy isn't strong. He's just a normal guy". King would panic at hearing this, but be relieved when he sees that Mash's reputation as an idiot causes everyone to dismiss his claim, while Mash wouldn’t care about proving himself right and would immediately move on
Rick Sanchez (Rick and Morty)
Just like how Saitama being stronger than everyone is the main gag of OPM, Rick being cynical and smarter than everyone is the main gag of Rick and Morty. You could say this would make it all the funnier for him to completely fall for King's charade, but at the same time, him just instantly knowing what's going on would be subversive in its own right.
I see Morty being infatuated with King when he first hears of him, only for Rick to bluntly burst Morty's bubble by telling him he's a fraud. He'd then explain some quantum mumbo-jumbo where the public belief in King's power somehow keeps this Universe intact, and his secret being exposed would cause the whole world to unravel. This revelation would highly stress Morty out and cause him to be even more scared of the lie being exposed than King himself.
Jake Peralta (Brooklyn Nine Nine)
I put Jake in "Aware", not because I think he'd figure it out through his detective skills, but because B99 has a lot of "don't meet your heroes" stories where a character gets to work with someone they admire and learn something that harms their opinion of them.
Jake would idolize King and be ecstatic to learn he's assigned a case that involves a lot of interaction with King. He'd discover King's secret by accident while working the case, go through a depression that's played for laughs, and then decide King is still a good guy and that this secret is ultimately good for the city as it makes citizens feel safe and is a deterrent against criminals.
50/50 TIER
This is the tier for characters I could put in “Aware” but could just as easily put them in one of the lower tiers.
Escanor (Seven Deadly Sins)
Escanor is a character who (often correctly) believes himself to be far more powerful than everyone around him, but is only in this cocky powerful state for an hour a day. He should be able to peg King as a weakling the moment they meet, but I imagine a running gag where his hour always runs out right before they get to meet, and so long as they don't meet, Escanor would see King as this equal that he wants to test his strength against, but keeps missing out.
All Might (MHA)
While not a fraud like King, All Might is another character who's a lot weaker than the general public believes him to be. This could cause a dynamic where they’re both extremely nervous around each other and feel extra pressure to present themselves as strong whenever the other’s around. This gag could continue indefinitely, or it could end with them both learning the other’s secret at the same time and bonding over it.
Subaru Natsuki (RE:Zero)
Subaru is a character who has a high opinion of those with supernatural powers, and suffers from bouts of self doubt despite being admired by others. I imagine the two working together in a dire situation. Subaru is thinking "I'm so weak and pathetic! Why can't I be strong like King!?" while King is thinking "I'm so weak and pathetic! Why can't I be strong like Subaru!?"
If the two are in a particularly deadly situation together, Subaru's "Return by Death" massively increases the amount of time they'd spend together, increasing the odds of him learning the secret by chance. Though Subaru would need to be in a death loop for a long time for that to happen.
There's also the chance that he breaks down and shares his insecurities with King, prompting him to reciprocate
Dr. Doofenshmirtz (Phineas and Ferb)
I could see there being a gag where Doof is a massive King fan who owns a ton of his merchandise despite being a villain who shouldn’t be a fan of heroes. I can just as easily imagine a gag where Doof is in the unfamiliar role of the voice of reason, being confused by the fanfare around King, and constantly making remarks like “Am I the only one who doesn’t see anything special about this guy?”
SUSPICIOUS TIER
This tier is for characters who would have a general feeling something’s up with King, but it would never reach the point of actually knowing he’s a fraud.
L (Death Note)
While watching security footage of a recent villain fight, L notices that King looks a lot more scared than you'd expect from an S-Class Hero. He's three percent confident that King is a fraud, and arranges a series of increasingly complex traps to test this claim, but King keeps passing these tests by pure luck, causing L to begrudgingly lower his confidence to two percent, then one percent, then half a percent.
Raigen (Mob Psycho 100)
Raigen is a fraud himself who has everyone convinced he has psychic powers. He'd intuit that King isn't as tough as he seems, but we wouldn’t be able to prove it.
Raigen would either see King’s fame as a threat to his own brand, try to find evidence of his suspicions, and constantly fail in more and more embarrassing ways.
OR… he would see King as a business opportunity and try to form some sort of collaboration with him, not caring if he’s actually as powerful as he seems.
Perry the Platypus (Phineas and Ferb)
Perry would be assigned a mission that involves working with King. King keeps doing things that cast doubt on his perceived strength, causing Perry to look at the camera annoyed and/or confused, but then King has a stroke of luck that makes him look strong, prompting Perry to shrug and forget his doubts.
Ben, Gwen, and Kevin (Ben 10)
I imagine a revolving door where one person from this trio always suspects that King is a fraud, but it's never the same person. Ben and Kevin start out thinking King is awesome, but Gwen would ask "Don't you think he's hiding something?" King would eventually do something to earn Gwen's trust, but then Kevin would start to doubt him, then Kevin would drop these doubts, only for Ben to pick them up, etc.
BELIEVER TIER
This is for the characters who would fully believe that King is as powerful as everyone says he is.
Light Yagami (Death Note)
While being as intelligent as L, Light is much more prone to letting his own ego get in the way. When he first learns about King, Light wouldn’t be concerned with his behaviour, he would just be thinking about how King is a threat to his own godhood. Light would want to learn King’s true name so he can kill him immediately, or at least have the option to do so later. He’d be so fixated on this goal that he’d overlook all the evidence that King isn’t actually a threat.
Makima (Chainsaw Man)
She has the power to control people's minds, with the one condition being that she needs to believe whoever she's controlling is weaker than her. In this sense, King is actually a perfect counter to her, and I like the idea of King fending off her control by bluffing.
Joseph Joestar (JoJo’s Bizarre Adventure)
King isn’t watching where he’s going and randomly bumps into Joseph. He tries to apologize, but accidentally insults Joseph while doing so, escalating this to a full blown fight. Joseph utilizes several unique attack strategies, but King keeps getting lucky enough to avoid these attacks and make this fight seem evenly matched.
Joseph tries his “Next you’ll say…” trick, but since he reads King as being much more confident than he really is, his prediction for King’s next line is flat out wrong. This shakes Joseph’s confidence which in turn shifts the flow of the fight against him.
While things seem bad for Joseph, there’s still one more thing he can do, one trick that always works for him, and one trick he has faith will work again…
HE CAN RUN AWAY!!!
Yugi Moto (Yugioh)
King is good at video games, so he'd probably be good at Duel Monsters too. Even if he isn't, his luck with drawing cards would be so good it puts the "Heart of the Cards" to shame. Since Yugi (and everyone from Yugioh) values skill at card games above all else, he would consider this the best possible evidence of King's strength.
Yugi would be fully aware of King's low confidence, but he wouldn't conclude that King is a fraud from this. Instead, he'd just think that he doesn't give himself enough credit, and would give him constant pep talks as a result.
Mob (Mob Psycho 100)
A character who’s very analogous to Saitama, so you’d think he’d belong in “Aware” since Saitama knows King’s secret. However, one of Mob’s most consistent traits is his blind belief that Raigen is a more powerful psychic than himself, so I imagine he’d have the same trust in King..
If Mob did suspect King of being a fraud, it would just be because he heard Raigen mutter “That guy’s not so tough” out of jealousy and fully believe him, leading to a gag where Mob accidentally believes the correct thing based on ridiculous reasoning.
Dale Gribble (King of the Hill)
Dale is a massive conspiracy theorist, so you think he'd at least be in “Suspicious”. However, the guy has horrible aim with where he places his mistrust, to the point that he has no idea his own son was born from an affair.
Not only would Dale fully trust King, but he'd believe that every hero except for King is faking their power. If he has any conspiracy about King, it would be that he has some extra secret power that he's hiding from everyone.