La Première Valkyrie

Rhea Vale — a baron's daughter obsessed with mechs — is stuck working as a janitor because women aren't allowed to be Knights. Her father tries to sell her off as a concubine to the planetary lord, and when her sister Elena takes her place and gets dragged away. When the substitute bride scheme falls apart, Elena puts her life on the line to buy her sister's freedom. Rhea returns — becoming the first female Knight in Federation history — and comes for everyone who ever hurt her sister.