Hatsumomo is the head geisha in charge of the Nitta okiya, and she makes sure everyone knows it. She's cruel, bossy, and manipulative, and as Sayuri astutely observes, "she was never happier than when she was about to make someone suffer" (15.1). Hatsumomo, why you got to be so mean?
Here's quick (dirty) laundry list of ways Hatsumomo is cruel to Sayuri:
And the list goes on. Unfortunately for Sayuri, Hatsumomo is the money-maker (and it's hard to shake your money maker in a kimono), so she has to be nice to her. As mother explains:
"We all of us think of only one thing—how we can help Hatsumomo be successful as a geisha." (4.43)
That's why Hatsumomo can get away with being the Mariah Carey of geisha. She's a diva because she can be. She has the power to act however she wants, because without her, the Nitta okiya would be nothing.
Is it possible that Hatsumomo is really lonely and just wants to be loved, and that's why she becomes a bully? No one ever considers this in the book, so we'll consider it for them. As we learn in the book, no one wants to become a geisha. Geisha have lost their families, and they're basically forced to be indentured servants/prostitutes. Why do you think Hatsumomo became the way she did? And does her past excuse her nasty behavior?
Sayuri asks us to step into her okobo (that's the Japanese word for wooden shoes), but she doesn't empathize with Hatsumomo. She portrays Hatsumomo as one-dimensional, single-minded, and cruel. The only nice thing she does to Sayuri is she puts a hair clip in her hair once. We're surprised she doesn't spit on her, because we're given no reason to believe she has a nice bone in her body.
Her one-track cruelty makes it hard to feel bad for her when Sayuri and Mameha spread rumors that Hatsumomo is "mentally unstable" (27.65). Hatsumomo couldn't run Sayuri out of Gion, so Sayuri gets her removed instead. Dang, geisha, that's harsh.
Only when Hatsumomo is gone does Sayuri consider that she has become Hatsumomo:
I sometimes lift the brocade cover on the mirror of my makeup stand, and have the briefest flicker of a thought that I may find her there in the glass, smirking at me. (27.101)
But she dismisses it, just like she dismissed Hatsumomo.