Here is the introduction of Violet, another frequent player in the early Peanuts roster.
Patty lasts in the strip longer than Shermy, but is later completely supplanted by "Peppermint" Patty. Schulz and Peanuts states that the Patty characters were both based on the same real-life person, with the second one being closer to her actual personality, although with decades between the characters' heydays it's easy to allow him some leeway in his depiction.
Violet leaves the strip as a major player a little later than Patty (if my memory holds up), but would occasionally show up even in the 90s.
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