Madoka Kuraki is a classmate of Ageha Yoshina. In the first chapter, it is revealed that she requested that Ageha take care of a former boyfriend of hers, who was stalking and harassing her. Ageha agreed to do this, and the next day, she says that her ex-boyfriend, Hondo, apologized, and promised to leave her alone. She thereafter offered him the 10.000 yen (Approximately 120 US dollars) as he usually charged to "solve problems". However, Yoshina refused the money, saying he was happy to help, and that he would help her out any time for free. She thereafter exclaimed her apparent joy, and asked if he could help find her lost kitten, named Mii, which he immediately agreed to do.
Although on the surface, Madoka Kuraki seems sweet, underneath, she is rather unkind and even manipulative at times. Despite the fact that she seems to have no interest in Yoshina, she seems to have no problem flirting with him in order to gain his assistance, whether for worthwhile tasks (such as chasing off her ex-boyfriend), or useless and time wasting ones (such as putting up posters to help find her kitten). Yoshina later seems to come to this realization, himself, when he sees her laughing with some friends of hers as they make their way from the garbage pile, and he overhears them laughing at someone whose wallet they stole and discarded. He then goes to the pile and sees that it is Amamiya's and returns it to her.
The next time Yoshina sees her, she asks him how the search for her kitten is going, and claims that she has been losing sleep over it. However, Yoshina seems to recall the incident of Madoka and her friends laughing at Amamiya's misfortune, and he blows her off rather brusquely, saying "I don't know. Maybe you should get out there and look for it, yourself," although his quickness in his response may partially have been because he was trying to get away from school before the fake detectives found him.
A good time later in the series, Madoka's unkind, and now, apparently, even mean, side is shown again, as she scowls at Amamiya and Yoshina, who are together, smiling and laughing, and she claims to her friends that she is annoyed by seeing Amamiya so happy, although given that almost the entire year, Amamiya had been cold, indifferent, and a recluse to the point of specifically requesting that her classmates not speak to her, it is somewhat understandable that they might find her sudden personality change odd.
Since then, she has not been seen again, and so, her character faded into the background, somewhat, as an unpleasant, but relatively inconsequential person, compared to many of the other antagonists, who all played a much larger role in the series. All things considered, her importance to the series was simply to establish the fact that Yoshino Ageha was a bit of a battle junkie, and so, she can hardly even be considered as a supporting role.