No, it really hasn't been revealed. I think it was heavily hinted at, but only confirmed as of vol 9 (of the LN) that he was bad. As to whether he's the biggest bad, the LN hasn't revealed that yet (the WN is different, of course, since that's finished)
@omegazion
The light novel and web novel don't have the exact same story, it diverged in this chapter. If you mean the translation you can find on novelupdates, those are the web novel version. @martinren123
same as zion, you read the web novel version, this is the light novel version.
@Al_Azif@jokerxhisoka You guys are right. I'm not talking about WN version since it's not quite relevant to the manga, but even in the LN version (prologue of volume 6), Yuuki's name wasn't mentioned in this chapter's particular scene. There was "the boy", but it wasn't revealed that "the boy" was Yuuki.
The whole conversation do take place in the LN, but Yuki identity wasn't revealed. He was just referred as 'the boy' and never actually name dropped. It was still suuuuuuper obvious who he was, but I guess those who're bad at paying attention got quite the surprise in this chapter.