So, I have been rewatching Chuu2 and this has been bothering me.
Who wrote that letter to Isshiki? Could it have really been Nibutani?
Why would she be interested in a mexican?
Because she was drawn that way?
It could have been just about anyone. As far as we know the letter was just a plot device.
It's a letter of challenge.
No, because "no love triangles"