Lydia Agthe serves as the sub-commander of Calamity Raven, the Imperial Army's elite unit, answering directly to its leader, Dahau. Approximately 22 years old in 1935, she stands 172 cm tall with blonde hair and violet eyes. Orphaned young, she was raised by the Yggdist religious organization, which gave her the surname "Agthe"—a name she dismisses as meaningless due to her lack of family. She demands to be called only by her first name. Unlike her unit comrades, she is not Darcsen, though her specific ethnicity is unknown. She wears a Valkyrian spiral pendant signifying her Yggdist faith.

Acting as Cardinal Gennaro Borgia's liaison, Lydia monitors Calamity Raven to ensure it serves Borgia's goals. Driven solely by self-interest, wealth, and power, she holds no loyalty to Dahau or ideals. She openly scorns trust, mocking those who value it, especially Darcsens. Cynical and sharp-tongued, she relishes tormenting others. An expert in mechanized combat, she commands the colossal Batomys-class tank *Echidna* and operates a personal heavy tank against Gallian forces.

Her dynamic with defector Gusurg shifts dramatically. Initially suspecting him as a spy, she assigns lethal missions to eliminate him. However, his moral flexibility and pragmatic dedication to his darker ideals intrigue her, evolving into a complex love-hate relationship. When Borgia betrays Lydia, sending an assassin to kill her and sever ties with Calamity Raven, Gusurg intervenes to save her. Facing disillusionment and lacking purpose, she accepts Gusurg's ultimatum to "die for him" rather than by his hand.

In her final act, Lydia leads a force to delay the Nameless squad, enabling Gusurg's plans. She acknowledges her role as a pawn, stating that if her life is meant for use and discard, she chooses to be used by him. After her tank is destroyed in battle, she dies seated on its wreckage, expressing hope for Gusurg's success, her arc concluding with resignation and fleeting satisfaction in her choice.