There’s a Peanuts movie out, and I understand they show the Little Red-Haired Girl in it. Apparently a cartoon or two has been done over the years where the LRHG makes an appearance as well, but this character was never once shown in the comic strip. Shultz himself didn’t consider those animated depictions canonical. What was he trying to tell us?
I suspect this is one of those Fight Club type deals, where it turns out the character is a projection of the protagonist’s subconscious mind.
I bet if I tried I could write a nice treatment on that theme, packed with Peanuts erudition and deep psychological angles.