Charlie Brown uses the term "pencil-pal" because he does not write well in pen. Normally, their communication is a one way, but his pen-pal is known to write back once in a while. When asked by Lucy about what they write about in August 28, 1958, he says, "He tells me about his country, and I tell him about ours...", to which Lucy responds, "You sound like a couple of spies to me." Charlie Brown's comment to Lucy indicates that he is writing to someone who does not live in the USA although which country his pen-pal was from was not revealed until September 27, 1994.
In August 29, 1958, 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.
The final reference in the strip to Charlie Brown's pen pal is in August 14, 1998.
In the musical, You're a Good Man, Charlie Brown, Charlie Brown writes a lengthy letter to his pen pal as the narrative framing device for the baseball game story. At the end, when he was about to write about how he was struck out in front of the Little Red Haired Girl, he decides against completing the letter and writes a new one that asks simply for directions for where his correspondent lives, fighting back the tears all the way.
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 only enabling Charlie Brown to communicate with her in a way that he feels comfortable, but it also allows for an unambiguously happy ending that does not significantly affect the Peanuts story world.