Tachibana Aya is a first-year student at Sohoku High School and a member of the tennis club. She doesn't know anything about bicycle road racing, nor does she seem to have much of an interest in it. She is a friend of Miki's.
Aya has short brown hair and often has a somewhat sour expression on her face with her lips drawn prominently and her eyes sharp.
Aya comes off as grouchy and unfriendly due to her dislike of otaku and her quickness to throw a punch at people who annoy her. In the anime, she immediately becomes a fan of Imaizumi, while in the manga, she regards him coldly even though he's Miki's childhood friend.
She often scolds Miki for getting excited about bicycles or being naive, but seeks revenge if people insult or mistreat her friend. She seems somewhat protective of Miki and concerned for her well-being, sometimes to the point of being over-protective. In the anime guidebook, she is jokingly described as "Miki's bodyguard."
Initially, Aya isn't interested in bicycles and has little faith in Onoda. She initially is concerned that Onoda is lying about his cycling abilities to take advantage of Miki's kind nature and love of bicycles. However, she follows Miki around to attend some of the races and witnesses his struggle. After the 41st Inter High, she's seen visibly attempting to be kinder to Onoda, though in a somewhat awkward and embarrassed way. Onoda, in turn, never remembered her name until second year.