And now the canon Letty shows her face. D: The one who actually doesn't want to be associated with Cirno at all! YOU MONSTER!
But really, Cirno's one of my favorite characters because I sympathize with her trying her hardest to be strong, but being completely outclassed by everyone around her (which is me in most everything physical). Her being slow in the head has little to do with it, though it was in fact the Odenpa Love Girl video that spurred me to get into Touhou in the first place.