Si Aen, a genetically engineered mecha pilot crafted by Hau Dragon for the Hakkeshu—eight elite robots designed to conquer worlds—operates in tandem with her twin sister, Shi-Tau. Together, they command a Hakkeshu unit defined by clashing temperaments: Si Aen’s icy detachment and emotional reticence collide with Shi-Tau’s seething jealousy and unpredictable volatility. Their fraught partnership, riddled with miscommunication and mutual frustration, cripples their combat efficiency, as Shi-Tau’s simmering insecurities flare under her sister’s unyielding aloofness.

Masaki Kihara, their creator, embedded fatal flaws in their genetic programming to guarantee their downfall. These defects erupt as chronic synchronization failures during missions, rendering them exploitable by adversaries such as Zeorymer.

In the OVA, the twins briefly surface outside battle in a hot spring interlude, reiterating their secondary antagonistic roles. Their story consistently circles their psychological entrapment—unable to transcend their engineered limitations, they stand as stark testaments to Hau Dragon’s imperious designs.