What a trip. This lady's out of this world, and not in a good way. She's like a symbol of all the sexual depravation and torturous cruelty that the characters in the novel have witnessed or experienced in their lives. Whereas they hide the memories behind brave faces, she hides herself behind false walls in a creepy palace of prostitution.
Madame Zhou is the one who betrays Lin, getting him thrown into the Arthur Road prison, as revenge for his rescuing Lisa from her palace. When he goes to get some revenge of his own, though, he finds such a pathetic scene that he can't even do it:
She was staring with hatred and spite at some point in the past, some place or event that held her mind as firmly as a chain holds a dancing bear. Her face was made up with a thick smear and powdering of cosmetics. [...] The painted mouth was bigger than her own lips. The scrawled eyebrows were larger than the real ones. The daubed cheeks were higher than the bones beneath them. [...] there was a trickle of drool dripping, dripping, from the corner of her mouth. (5.38.50)
Madame Zhou is not dead yet, but she's on her way, and her appearance is a metaphor for her life. Everything is painted on to seem bigger and scarier than it really is; she provokes more fear by hiding herself than by ever actually doing anything. That's the real source of her control.