Aona Day is the older sister of the main protagonist Kamon Day and his younger brother Ryoma, also known as Roma. She lives and works in Crest Land, the central setting of the series, where she runs a restaurant called AONA. Her appearance is characterized by dark teal blue hair of medium length, which she often holds back with a pale pink headband, and teal eyes. She is a little taller than her brother Kamon and typically wears an elbow-length sleeved layered jacket, yellow over orange, with a white collar and laces, paired with a blue dress and peach-red boots.

Aona's personality is defined by her caring and protective nature, particularly towards her brother Kamon. Her primary motivation stems from a past, fateful event in which Kamon lost his memories after a defeat at the hands of their other brother, Ryoma. Following this traumatic incident, Aona took it upon herself to look after Kamon, driven by a deep fear that the two brothers might fight again, which she believes could lead to Kamon being killed. This protective instinct manifests as an attempt to keep Kamon away from B-Daman battles, as she associates the sport with the danger and trauma of his past.

Within the story, Aona serves a crucial supporting role. While she runs her restaurant, she is not a battler and does not participate in B-Daman fights herself. Her position is that of a guardian and a stabilizing presence, especially as Kamon rediscovers his passion for B-Daman and his partner Garuburn. She is also the daughter of Greg Day, the owner of the B-Junk B-Daman shop, who eventually reveals himself to be Kamon's father. Her family connections are central to her character, and she shares a good friendship with the WBMA referee, Rory Takakura. Another character, Yuki Washimura, is noted to have a crush on her.

Aona's development is closely tied to her evolving relationship with Kamon and his journey. Initially, she is resistant to his involvement in B-Daman, fearing the potential consequences of him remembering his past and confronting his brother. However, as the series progresses, she begins to support him, even joining him and his friends on significant adventures, such as the trip to investigate the mysterious Phoenix Battle Island. Her actions show a shift from trying to shield Kamon from his past to standing by him as he faces it, demonstrating her care for his well-being in a more active and supportive way. Aona notably lacks battle abilities, as she does not own or fight with a B-Daman, making her contributions to the plot entirely non-combative, focused instead on emotional support and family dynamics.