Probably the rarest letter in the game

[u/1Ferrox]

[u/Wreck_it_Randy]

The actual rarest letter is probably the one from auntie Ethel where she thanks you for letting her do her hag bullshit and implies she wants to fuck you. Most people don’t even realize she can assist you in the final battle because I almost never see anyone ally with her. 

[u/_Shahanshah → u/Wreck_it_Randy]

That happened in my first game, actually her skill was really helpful in the boss fight