Mc isnt that dense he said it himself that it would make sense that fuyuki and natsumi is close if she was the childhood friend of theirs but different first names? He just discards the option, the real problem is natsumi just tell him who you r and confess. Thats why these type of stories feel like shit to me, bc there is actually nothing worthwhile as a story if it can be solved with just a sentence. To make a story author creating unnecessary misunderstandings and drama which is annoying most of the time.