Akua Aino is a supporting protagonist and Seiji's younger sister.
Seiji's tomboyish younger sister. During their childhood together, Akua was originally far more girly than her present age. She began to change herself however when Seiji began spending less and less time with her in order to get his attention back. Despite this, Seiji failed to notice her to the point where it developed into animosity against Seiji, much to his confusion. Upon discovering Seiji's newfound harem, her discontent grows further. Akua tends to injure Seiji whenever she is irritated with how ignorant Seiji is about her emotions.
She is being hunted by Stolas.
Like her brother, Akua has short blue hair and blue eyes. She appears to be 13-15 years old. She usually wears her school uniform with a vest on top and leggings underneath. She also wears white socks ankle-length with red sneakers and red wristbands. She is also known for her boyish style.
She has a brother complex. She somewhat acts like a tsundere with her brother, but loves him unconditionally. She is really judgmental as shown when she discovers that Seiji is in a harem, she starts to disliking even more.
Though feminine in her early years, she becomes more of a tomboy as she grows older in an attempt to attract her brother's attention. While she deeply loves her brother and wants his attention, it is unknown if it is because she simply wants a close relationship with her brother or may actually have romantic feelings for him.
- "I Can Handle This Myself! x Wassup"
Aqua learns her broth er has 3 girlfriends and punishes him. Yuzu attempts to befriend Aqua but Aqua accidentally reveals her own feelings for Seiji. They are approached by Stolas, a demonic penguin obsessed with Aqua. Seiji saves Aqua while Stolas is arrested.
- "Go to the Beach With Me? x It's Not a Matter of Knowing or Not"
Seiji invites his sister and the girls to the beach. Stolas, who has returned for Aqua, and is badly injured.
- "Farewell x We're Rivals, Obviously!!"
As Seiji was about to be killed, Akua knocks out the assassin and saves her brother, h er and Seiji went to the Hiyama household. Later, Akua is seen fighting and defeating the bodygrounds easily, when Shikimi throws Coraly at Akua making her pass out.