Sayuri and Pumpkin
It's kind of sad to see the fate of these two friends (practically sisters). However, envy and jealously tore these two apart, which is understandable, especially from Pumpkin's perspective. She envied Sayuri. To Sayuri, things came so easily for her, whereas it takes so much effort on Pumpkin's part.
I'm not sure if this was explained in the book, but it was unclear if Pumpkin remains in contact or even friends with Sayuri. I assume, based on other comments, that she unfortunately went back to the life of prostitution, but did Pumpkin ever forgive Sayuri? .....Ever? Or was the friendship a fake until she found the right time to sabotage her? Was Pumpkin pretending to be friends with Sayuri ever since Sayuri was named the next inheritor of the Okiya?
It seems like Pumpkin was always jealous from the beginning but hid her feelings.