Corporal Mayuka Nasu serves aboard the Free Pact Alliance warship *Cygnus*, primarily assisting scientist Dr. Elvira Hill with the Coupling System while also performing bridge duties like radar operation and communications. She wears the standard pink Alliance uniform for her rank. Her physical appearance features a notably short stature, auburn or light brown hair styled with two thin face-framing bangs, and dark pink or aubergine eyes. During shore leave, she switches to casual attire: a yellow skirt and blazer.

Characterized as soft-spoken, reserved, and timid, Mayuka approaches situations cautiously and reflects deeply on ethical decisions. She forms close friendships with fellow bridge operator Anessa Rossetti and Dr. Hill, contrasting with Anessa's energetic nature. A significant aspect of her personal journey involves unrequited romantic feelings for Aoba Watase, the time-displaced high school student recruited as a Valiancer pilot. She worries intensely about his potential departure from the *Cygnus*, becoming visibly distressed when he is missing in action and crying in relief upon his safe return. Her attempts to connect with him—explaining the war's history or encouraging him to remain with the Alliance—highlight her optimism and loyalty to the crew, though she is disheartened when he discusses other romantic interests.

Her responsibilities include escorting Aoba upon his arrival on the *Cygnus*, briefing him on the conflict between the Free Pact Alliance and Zogilia Republic, and later supervising his visit to Narashino Base. During this tour, she persuades him to embrace his role as a pilot after he expresses reluctance. She participates in crew activities like shore leave in Hawaii, assisting Anessa and Dr. Hill with shopping. Deeply affected by Aoba's return to his original timeline after the war, she later catches the bridal bouquet at Dr. Hill's wedding to Lee Conrad, symbolizing a forward-looking conclusion to her arc.