Materia Shimai are twin Frame Arms Girls designated as White (Shiro) and Black (Kuro), sister units sharing identical models. They possess an intensely affectionate bond, consistently displaying physical closeness and deep mutual devotion. Their personalities exhibit sadistic tendencies; they delight in psychological torment, frequently targeting Stylet for her tsundere reactions and occasionally teasing Ao. They provoke and manipulate their chosen targets to cause deliberate discomfort.

In combat, they intentionally forgo standard armor, deploying overwhelming heavy weaponry instead. Their tactics rely on perfectly coordinated attacks and seamless teamwork, rendering them nearly invincible opponents. This capability was demonstrated when deployed alongside other Frame Arms Girls to Ao's residence, where they primarily battled Gourai. After Gourai's victories, they remained at Ao's apartment within the expanding group.

Their emotional range remains minimal outside their sisterly connection; expressions of joy stem almost exclusively from observing or inflicting discomfort on others. Interactions beyond their partnership are rare, reinforcing their isolated dynamic within the group. Manufacturer Factory Advance provided no elaboration on their origins or specific operational purpose beyond standard data-gathering functions.

Later appearances, like in the recap film, show them participating in group activities wearing school uniforms and offering commentary on past events, though their core traits persist unchanged. No significant evolution or background expansion occurs regarding origins, internal mechanics, or relationships beyond established patterns. Their narrative function centers on antagonistic comedy through psychological manipulation and showcasing high-powered combat tactics.