Sayaka is very good at cunnilingus.
She claims to really like Asumi and gets little floating hearts around her head when they hug... but, every time they get together, after sex, she's like: "Bye-bee, Asumi-chan! If you wanna meet again, you know my fee!" This means she's not really romantically interested in Asumi -- unlike a girl like Serina, who seriously told Asumi she was willing to stop the prostitution job and become her girlfriend.
Then again, we can't expect Asumi to make the whole brothel fall for her, haha.