Aiko Aoki is a young character who appears in the third season of the anime, introduced in the Present Era arc. She is a sixth-grade elementary school student known for her responsible nature and is the younger sister of a boy named Yuki. Her family home serves as a central setting during this story arc, where the immortal protagonist Fushi and other companions reside.

In terms of personality, Aiko is depicted as a cheerful and sweet girl. She is delighted to have Fushi and his friends staying in her home, finding the atmosphere more enjoyable and lively with their presence. Her responsible attitude for her age is noted as a key trait, which aligns with her role as an elementary school student managing her daily life alongside the extraordinary events unfolding around her.

Her role in the story is primarily as a supporting character within the Aoki household. She is part of the new group of characters that Fushi encounters in the modern era. Her key relationship is with her older brother, Yuki, and their grandfather, Kazumitsu, as they all live together. She also forms a positive connection with Fushi, welcoming him into her family's daily life. Notably, she finds the immortal endearing, especially when he takes the form of a dog.

Regarding her development and notable abilities, the available information does not provide specific details on character growth over time or any special powers or skills she may possess. Her significance lies in her function as a friendly and warm presence within the new setting, helping to ground the fantastical narrative in the familiar environment of a contemporary family home.