A disclaimer up front: it's a bit late & my brain decided it was ready to shut down, so if some of this reads poorly I apologize. As to your actual ??: I think the consensus is we're like 99.99% sure that it's Takiya; the theory is basically that he was actually the young boy in that Isekai side story a while back, & managed to retain his (probably?) Hero-level powers once he got back to the "normal" world (or at least they seem to've returned at some point since.) Some bits of evidence:
1.) I'm terrible w/ names, & esp. JP ones, but for starters you can (apparently) rather easily form the name that kid gives the king out of some bits of Takiya's full name.
2.) More importantly, there's been quite a few background and/or "non-verbalized" events we've seen in-panel that appear to either reference bits of the events of that side story (biggest one I can recall right now is that, during Elma's marriage arc IIRC, he appears to be apologizing to Elma's Grandma (Telne?) about cutting her horn off in the background of at least one panel) or link Takiya to details of the Masked Man himself (the recent panel where Shota appears to be asking Takiya for fashion advice, much to Lucoa's confusion, & shortly thereafter Shota explains his recent behaviors were bc he wanted to be able to contribute in the increasingly frequent major battles; I think part of the discussion was that he was attempting to learn HOW to successfully contribute, but he also lays out that he's trying to create/acquire a disguise so that he can remain anonymous while doing so, with an added descriptor that was something to the effect of "like the masked man!") I feel like there's a much longer list of these scenes than I can currently recall, but these are the 2 that spring to mind immediately & IMO seem the most pronounced.
I reserve that last .01% for the off chance that Cool is just trolling us & all of these apparent references are an intentional Red Herring. I don't really think Cool has a rep for that or anything (& I've read a fair bit of his stuff at this point), but the general tone of his writing output (which I'd describe as rather playful on the whole) makes me feel like it's VERY possible that he'd pull off something like this for a bit.