He knows in the serialization not the one-shot.
However, he seems to consider that she has a harem, is naturally very flirty and way out of his league.
I think that Itsuka like/love Shayuu but :
1. He believes her to be a sort of easy play"boy" and thus here efforts to win his affection are read as her normal behaviour with everyone.
2. Even when he doubt it's the case, he's depreciating himself, and considers her The Hero and thus inherently...
By the way, rereading the series, I was wondering from where the images ar the very last page of chapter 5 (the announcement from the translation team) came from.
They don't seem to be from previous chapters or the One Shot.
Also, I was just thinking: if author goes toward Itsuka and Shaayu...
Okay, so, crazy theory time now that the story is getting somewhat serious again.
1. Shaayu only started falling for Itsuka a month ago.
This one is only a theory because the manga hasn't explicitly confirmed it, but the start of chapter 2 (I think) gave us a huge hint in that direction, where...