Aki is Haru's younger sister.
Aki is selfish and wants to take Haru's place as heir to the Jigokumeguri Zaibatsu. However, she hide this behind a persona of being a sweet and loving younger sister. Similar to how Haru has trouble being nice, Aki has trouble being mean and often ends up helping Haru by accident.
In the past, she was incredibly close to her sister.