"I'm on the good guy's side! And I practice karate everyday 'cos of that! If I wimp out and hide now, I'll never be able to forige myself."
Aya is a young girl with red hair that is worn in a bob with hairpins on both sides of her hair.
She wears a tan duffel coat with two off-center fastenings. Underneath her coat, she wears a yellow dress with violet tights and pink shoes.
Aya is a spirited girl who calls herself a warrior of justice and practices martial arts every day. She is stubborn, refusing to take no for an answer, and makes fun of Doppo Kunikida often. She seems to value helping others and is willing to sacrifice herself for the sake of others.
While in the train station, Aya receives a suitcase from a unknown man, assuming that it is a lost item, agreed to take it. Kunikida then acidentally saw the suitcase and remembers what Tanizai about suitcases containing bombs. He then takes the suitcase from her, to which he opens it and it contains bombs. Kunikida throws the suitcase at the air saving her and the others in the train station.
Aya then became angry at the man who gave him the suticase and decides to help Kunikida find the culprit and goes to search with him, even dragging Kunikida on his own. Both of them are in a train tunnel to which the culprit went. When Kunikida was busy looking at the bombs strapped on the rails, Katsura manages to strap a bomb to Aya, taking Kunikida by surprise. Having to choose between saving Aya or saving a train from being blown up, Kunikida apologizes to Aya. She accepts her fate, saying it is for the sake of everyone else, and the two of them are hit by the bomb.
Yosano manages to heal the two of them, so both Aya and Kunikida are unharmed. After Katsura is captured by the police, Aya implies that she wouldn't mind being Kunikida's girlfriend. Kunikida tells her that she doesn't meet his requirements for a partner, turning her down gently.
- "I'm on the good guy's side! And I practice karate everyday 'cos of that! If I wimp out and hide now, I'll never be able to forgive myself." 
- The name Aya means "sentence, letter, writing" (文).
- Aya's surname Koda means "happiness, good luck" (幸) (ko) and "field, rice paddy" (田) (da).
- Aya speaks in the Kansai dialect.
- Prior to the anime OVA, Aya's family name was unknown. The anime revealed that she is also based on an author, Aya Koda.
- Bungo Stray Dogs Manga: Chapter 40.