The Pie Sisters represent the classical temperaments, which according to the ancient Greeks stemmed from an excess of one or another of the four “humors” which flow through the body.
 
Pinkie Pie, “Sanguine”: Stemming from an excess of blood, this temperament is happy, outgoing, and energetic.  
Limestone Pie, “Choleric”: Stemming from an excess of yellow bile, this temperament is passionate, aggressive, and impulsive.  
Marble Pie, “Melancholic”: Stemming from an excess of black bile, this temperament is passive, withdrawn, and prone to depression.  
Maud Pie, “Phlegmatic”: Stemming from an excess of phlegm, this temperament is passive, calm, and unemotional.