[ He remembers that question Yato once asked a while ago, on a forum, about having a dark past they've hidden under a bright façade. The conversation had made it apparent that Yato was referring to himself... and now Ratio was finally getting the chance to see part of the past the god was trying to hide. Not that he'd wanted to see it this way, though. Such things should always be disclosed at one's own discretion. But as usual, none of them have any choice here. ]
2; probably after the kaiju fight
So this was what he was alluding to.