Catherine Weller is an American combat pilot assigned to the 307th Aerial Squad, designated "Hell Alice." Her first appearance occurs during a rescue mission led by Aria Hitotose at Midway Base, devastated by an alien Pillar. As the mission's sole survivor, Catherine suffers amnesia from the trauma. During the return flight, she bonds with Aria, inspired by radio chatter referencing Aria’s "Ice Doll" moniker and sensing her compassion. Catherine adopts her name "Catherine Weller" and enlists in Hell Alice. She flies a McDonnell Douglas F-4 Phantom II with markings akin to the USAF's 178th Attack Squadron, using the call sign "Cathy."

In the in-universe anime *The Third Aerial Girls Squad*, Catherine becomes one of Aria’s closest friends. The original manga depicts Catherine’s death in action, causing Aria to abandon flying. The anime adaptation diverges: after Catherine’s death, Aria travels to her hometown and encounters Lucy, Catherine’s younger sister. Resolving to protect Lucy, Aria resumes flying. Lucy Weller, voiced by Shizuka Sakaki within the *Shirobako* narrative, is an anime-original character. Her inclusion shifts the ending toward Aria’s emotional recovery through commitment to Lucy. Catherine thus functions as a fallen comrade, indirectly catalyzing Aria’s development and Lucy’s introduction in the anime.