The only thing I can see this being is he doesn't exist, he is saitama, or simply he just is a new character. Only reasoning is what if saitama has like amnesia and simply made up a past or what if he is hiding it from everyone and acting clueless. Then there is the chance he doesn't exist, I mean we have seen certain people say they remember him but what if that was just some psychic fucking with their heads so everyone trys to grow stronger and become the new no.1(like what beerus did to goku) and this would also make everyone think there is hope. Also no way Orochi is him I mean gyoro gyoro said Garou was stronger than Orochi when Orochi was a human and Garou isn't strong enough to be no.1 yet. My last weird theory is that he is either Saitama in the future or a family member of Saitama I mean think about it we don't know if saitama has family but we also don't know his limits. What if saitama will gain the power to go back in time and he knows of a threat coming in the future of the anime and knows his current self in that time wont be able to defeat it or something like that that already has happened, or maybe he is saitama and also the evil cyborg knowing that he needs Genos or something so he became the cyborg that destroyed his village I mean he could really be anyone but for some reason I think of that all the time. Just a smart future saitama because why not.

... Dude, do you even read the original webcomic? He CLEARLY exists as it's shown there.

That crap about him coming being a future Saitama is just so cliché and unoriginal not even ONE could spin it into something interesting.

In short, My theory, Hes dead... I think a time will arise when an actual threat to Saitama Villain will appear having announced he destroyed Blast a while ago, OR! The theory I wish to be real, Mumen Rider is Blast, Yes theres a lot to disprove that but imagine Blast lost his powers and is still trying to be a Hero, or maybe Mumen’s Pride comes from possibility that Blast is his Brother, So he strives to be just as good a Hero however he can, Mumen Rider and Blast general looks, Hair, face just look so near Identical!

Im thinking theres a good chance it still could be Saitama, Based off flash back of him years ago I could see it being Saitama before he lost his hair in first suit he ever made and wore and he even told her being a hero was more of a hobby for him, Sound familiar?

Fact that he didnt appear at hero meeting while Saitama was there, Maybe there was a good reason for Saitama abandoning his old life as Blast, Doesnt care for spotlight obviously, The way he stepped in as Blast to OP Centichoro? Saitama is stronger than Blast if Blast couldnt finish Centichoro in an entire fight let alone a single punch, Would make sense Saitama is a lot stronger than he was back then, Whatever made him stronger that lost his hair since his first encounter with Centichoro Im not sure we’ll ever know the truth to Saitamas power or to Blasts true Identity if it even exists but its rather odd when ya piece a lot if it together....

Blast is saitama...

Blast is saitama from the past because, saitama was the reason why heroes association was created. And even though, when heroes association was created saitama had hairs on his head...3 yrs ago.

Saitama defeated monsters and the heroes association didnt know name of saitama. Since, saitama defeated monsters with a blast, they named him blast...

They cosidered him as the strongest hero alive and made him class S rank 1 hero even though he had not registered the hero association...

And now, in the present period saitama doesnt have any hair left so, they dont recognize him...

And the thing left with the elder centipede,

Saitama was not that strong to kill the elder centipede 3 yrs ago but, he injured him badly...

The elder centipede got away that time...afraid of saitama(blast)...saitam a always forgets anything, so, he forgot that he had met tatsumaki once..

Hence, saitama is blast from past, not from Future...

SAITAMA=BLAST

Blast have a son and his name is Blue, he would atleast recognize Saitama if the Saitama we know is the past version of Blast (IF Blue time traveled) just like how King recognized Saitama to be the civilian who saved him once.

I think blast is saitama from the past as they are the only ones capable of fighting the elder cenripede