Not sure if this has been posted already, I looked around and saw nothing so I went ahead.
I think most of us, if not all, guessed that it was the fact that Ray is Isabella's son. But I'm quite surprised to know that apparently no one has a correct guess (as far as Shirai knows, at least). Which implies that maybe Ray actually knows more secret than what he told. It's nice to know that Shirai will reintroduce this plot point later, because otherwise what's the point of chapter 22 then?
For now these are clues we have:
- It was something Ray knows, but likely he shouldn't know it under normal circumstances
- It was something Isabella shouldn't know
- It could threaten Isabella's status as a Mama if the higher-ups know it (Yet Grandma didn't look surprised after reading the note. Did she know that already?)
- It was something that can be confirmed through Krone's files
I checked chapter 28 to find some more hints from Ray's early memories, but can't find anything concrete. I'm curious now.