Maybe the dreamer loved someone a very long time ago who commit suicide and after that, kept dream unconsciously about how her life would be if she confessed to her and were her girlfriend.
I'm pretty sure I already saw a manga about a guy crossdressing into his sister to keep boys away or something like that once, but well, it stays something pretty rare to see.
But for sure, the random boy with the "I was the only one" line is so cliché.
I think it's Nanashi no Asterism the manga you're talking about.
