Yumi Ikoma is the daughter of Professor Masayuki Ikoma, architect of the top-secret Plan L deploying Fafner units as humanity's final defense against the Festum. This heritage places her at the mission's core, though she initially grasps little of its scope.

Recruited as a pilot for the experimental Fafner Titan Model (TSX), Yumi learns her role only after her father's death. Discovering his handheld device containing Plan L data and an Alvis keycard leads her to the Fafner hangar, severing her ties to ordinary life on Tatsumiya Island and compelling her conscription. Her drive fuses duty to her father's legacy with unresolved grief, fueling conflict with fellow pilot Ryou Masaoka, whom she partially blames for her father's demise.

Aboard the decoy vessel L-Boat during Plan L, Yumi and fellow pilots defend the drifting segment to shield Tatsumiya Island from Festum attention. The mission's psychological burden intensifies as pilots fall victim to assimilation—a lethal consequence of Fafner synchronization. Yumi pushes for reduced combat time to mitigate this risk, revealing her developing leadership and concern for her comrades. Her relationship with Ryou shifts from resentment to reliance; she first rejects him as her synchronization partner but later acknowledges his vital support. In their final battle, they remain behind to prevent Festum from tracking the escape submarine back to Tatsumiya. Submerged in their last moments, Ryou confesses his love. Yumi expresses uncertainty about her feelings yet affirms solace in his presence. She questions whether their sacrifice would win her father's approval before marine snow envelops their Fafners, symbolizing communal remembrance.

In later series, Yumi appears posthumously during a vision sequence in *Exodus*, manifesting before characters Kenji Kondou and Sui Kaburagi, reinforcing her enduring symbolic presence within the narrative's themes of memory and loss.