"We're really happy being allowed to be with you, Nanami-sama."
For every group, there must be a leader. For every leader, there must be followers. The one in charge instructs and commands the followers. They obey. Such is the nature of things.
Obviously, the fabulous Miss Nanami Kiryuu is the dominant one. The queenbee, if you will. She has two sets of followers: three young men named Tanaka, Yamada and Suzuki, who are by far her biggest admirers, and three girls, all her age, named Wakiya Aiko, Oose Yuuko and Sonoda Keiko. There is also Tsuwabuki Mitsuru, Nanami's personal slave, but that's for another page.
Now, let's concentrate on Keiko. As you can probably see, she seems to be the dominant girl in the female faction of Nanami's servants. Having that position, she will often be the one who slaps Anthy Himemiya around (Anthy-haters, rejoice!) and leads the plots which Nanami instructs the group to do.
But why go through all that? Why take orders from the Kiryuu brat? Simply because, like Aiko and Yuuko, Keiko has a crush on Touga. Serving Nanami and doing all her dirty work for her is Keiko's way to get close to him. Although, there is no solid evidence that she really does get very close to him, yet once, she got to share an umbrella with him (see "Spotlight").
Now, you know why Keiko, no matter what, puts up with Nanami. In the words of the Rose Bride, Anthy Himemiya, "For someone you love, your feelings for any other become insignificant...you can deceive yourself as much as you need." To put it simply, you'll do absolutely anything to be with the one you love and care about...