Tachibana Aya is a second-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. She is characterized by her short temper, easily becoming annoyed or impatient. 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.
Despite often becoming put off when Miki becomes excited talking about cycling, she still often listens to her. She usually scolds Miki for being naive or going on with her "alien bike talk," 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 witnesses his growth over his first year and hears about his victory at the 41st Inter High from Miki (or in the anime, witnessed it for herself). 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.
Aya seems somewhat more relaxed and friendly in her second year, though her grumpiness and temper occasionally still flare (such as when overhearing Inter High spectators speak badly about Sohoku's team). She often complains to Miki about the amount of work they have to do as members of the support team, but she still pulls her weight.
As the member of the support team with the least knowledge of road racing, she often fills the audience's role of asking about the more esoteric aspects of cycling or the Inter High . At times she seems concerned about how hard the members of Sohoku's cycling team push themselves.