Hisuka Aiheap, a human Imperial Knight of Zaphias, serves in the Niren Brigade alongside her younger twin sister, Chastel Aiheap, under Captain Niren Fedrok. The twins share identical red hair and yellow eyes, though Hisuka possesses a flatter chest. Hisuka exhibits a stubborn and stern demeanor, contrasting sharply with Chastel's gentle nature. This personality difference defines their roles; Hisuka is specifically tasked with mentoring and managing the impulsive rookie Yuri Lowell, frequently admonishing him for recklessness during missions.

Her duties involve active deployment against abnormal aer disturbances near Ceazontania. She participates in critical missions, setting blastia traps to combat monster infestations and investigating the aer anomalies' source. During one mission, she activates a Force Field blastia to protect her team while Captain Niren triggers a destructive blastia against a monster horde. Her responsibilities include tactical operations, such as defending Ceazontania from aer-formed monsters alongside Yuri. This mission results in Yuri being forced to kill Niren's enraged hound, Lambert, to protect her.

Throughout the Ceazontania events, Hisuka remains dedicated to the Niren Brigade. She witnesses Captain Niren's sacrifice during the investigation of an aer-manipulating fortress, where he perishes ensuring Chastel's safety. Following Niren's death and the exposure of strategist Garista's betrayal as the cause of the aer crisis, Hisuka continues her service. After the town's evacuation due to failed barrier blastia, she remains with the Imperial Knights while Yuri departs the organization.