Man it had been a while.
But I remembered now. My theory is Hayasaka actually liked MC from the beginning. She watched their soccer practice for him, not the senpai. But when MC asked if she liked senpai, plus she knew MC liked Tachibana, she had to say MC is second.
So, the letter guy is probably right.
Edit: Btw, why are the chapters often ends with cliffhanger but no continuation in the next chapter? Is there no part 3?