In one strip, Charlie Brown writes in one of his letters that his "pencil-pal" is his only friend, with the postscript "Everyone hates me".
In a series of strips from September and October 1994, the pen-pal is revealed to be a girl from Scotland named Morag. Charlie Brown fantasizes about a future romance with Morag, but his plans are crushed when he learns Morag has thirty other pen-pals.
In Snoopy, Come Home, Charlie Brown reveals he had fifteen different pen pals, but he ended up doing all the writing and got no replies.
In The Peanuts Movie, the Little Red-Haired Girl becomes Charlie Brown's pen-pal at the end of the story, not enabling Charlie Brown to communicate with her in a way that he feels comfortable, but also allows for an unambiguously happy ending that does not significantly affect the Peanuts story world.