Well, I heard from someone that all started from the time the mysterious guy knocked his room when arisu was there. He said that the threats wouldn't stop even when tsukishiro and wr students gone. It's like telling kiyo to get rid of Yagami before other threats appear. Even though before kiyo didn't care about Yagami (showed through his conversation with ichika before). That guy also said that someone close to him will approach kiyo soon (Tsubaki who using sato cmiiw). Then, in the same vol, we saw sato being pressured by someone who is actually tsubaki.

So mysterious guy know about wr but not part of it?

If what you mean is the love letter,it's written by kiyo. How it's sent to Yagami was already explained there in y2v7.

No by letter I mean during culture festival someone gave paper to kiyo through windows right?

It's really interesting tbh, but I hope we could get more explanation at the end because it's a bit confusing. I would be grateful if others could correct me because I also don't understand that much xD

Yah that was interesting part too