Responses
It has been implied but not revealed (so this is an educated guess) that when our hunter ML first manifested(?) the Yoon bastards saved him and Hayeon or whatshisface claimed to be the one to purify/help him, but I'm pretty sure it was our purifier MC who first purified the young ML. This is also the reason why ML thought MC's power feels familiar/nostalgic.
What does "that day" means? The day Haeryung got SA?