Ayaka Machida (町田 初佳 Machida Ayaka?) is one of the "Big 4" of Stellvia, (older students who are at the top of their classes). She is shown as competitive in the extreme. She has something of an inferiority complex which drives her to be the absolute best, and she can't tolerate seeing others better than her.
Two years in the past, when Yayoi was her classmate, she arranged an accident to ensure her own position at the top of the class. Things went a little out of hand, however, and it looked like Yayoi might lose her life, so Ayaka rescued her. Yayoi was hospitalized and traumatized, and left Stellvia for a time. Now with Shima Katase looking like she would surpass Ayaka as well, the same thing almost happened. Had Yayoi not returned to Stellvia and recognized the danger Shipon might be in, Ayaka might have seriously injured or even killed her. Due to Yayoi covering up the truth of the earlier incident, Ayaka was not expelled from Stellvia, but was suspended for a time.
After Yayoi forgives Ayaka for nearly causing her to lose her life, the two become very close.