Ayu Tateishi has long, brown hair and oval grey eyes. She is usually seen wearing her school's uniform, which consists of a blue cardigan, red tie, white dress shirt underneath the cardigan and a plaid red skirt with navy blue socks. Occassionally, she is seen in a white sleeveless cardigan with a white short sleeve dress shirt underneath with the same colored skirt, and white, loose socks. Away from school, she usually dressed in stylish outfits, some even lounging outfits.
In the anime, it is shown that Ayu is taller than Nina, setting about 5'4 or 5'6.
In the beginning of the manga, Ayu is shown to be calm, cool, and collected. The reason for that is Kaji Tetsushi praised Tateishi for her mature personality when the teacher explained to the rest of the students that there will be a test, a part in which Ayu missed due to daydreaming. When he told her that he like that about her, Ayu wanted to keep up that mature side so Kaji could like her.
Ayu explains to Nina that she began to act more calm and adultlike, started focusing on what she wore, and was picky about even the smallest things and not get too involved with the other girls. With her mature personality, other girls idolized her and that's where her popularity began to soar.
When she began hanging out with Nina, her personality began to expand, she can be very caring and a good friend, but can also be worrisome and a bit reckless and bossy sometimes.