Birdy Cephon Altera is a bio-engineered Ixioran supersoldier from the planet Altair who serves as an investigator for the Space Federation Police. Her duties involve pursuing interplanetary criminals across galaxies, leading her to Earth while tracking fugitives. During one mission, she accidentally fatally injures a human high school student named Tsutomu Senkawa. To preserve his consciousness during his body's reconstruction, Birdy merges him into her own form, resulting in a shared existence where both personalities inhabit one physical vessel and must coordinate their lives.

Her physical capabilities include superhuman strength sufficient to lift vehicles and fracture concrete structures, enhanced speed, agility, reflexes, and durability allowing her to withstand ballistic impacts. Her senses are heightened, particularly her olfactory abilities, which she uses for tracking targets across long distances. She possesses a transformation ability, enabling her to alter her appearance or switch between her form and Tsutomu's to maintain his civilian life. One specialized technique is "Crash," an electrically charged attack delivered through physical contact. Under extreme stress or via external command, she can enter a "Q-body" state, becoming a nearly indestructible monstrous entity with amplified destructive power.

Birdy's personality combines energetic assertiveness and tactical bluntness with a deeply ingrained sense of justice. She exhibits occasional social awkwardness and impulsiveness but demonstrates acute deductive skills during investigations. A core aspect is her commitment to preserving innocent lives, even at personal risk, which sometimes conflicts with her role as a law enforcer when confronting morally ambiguous threats.

Her backstory includes significant childhood trauma from a terrorist attack on Altair's Central Tower. Her caretaker, a marionette named Violin, sacrificed herself to save Birdy from a Berserker unit. Birdy subsequently destroyed the unit in a rage, earning the moniker "Berserker Killer." This event left lasting psychological scars that resurface in adulthood when confronting violence or loss. In some continuities, she was rescued during this incident by her childhood friend Nataru Shinmyou or by Federation officers.

In "Decode:02," Birdy grapples with the aftermath of the Roppongi district's destruction, an event tied to earlier conflicts. Her relationship with Nataru becomes central; he emerges as a vigilante targeting perpetrators of the Roppongi disaster, employing brutal methods that contrast with Birdy's adherence to lawful enforcement. This conflict forces her to confront unresolved trauma and ethical boundaries, particularly when Nataru's actions escalate into personal confrontations. Their dynamic explores themes of justice, vengeance, and shared history, culminating in physical and emotional clashes.

Across different continuities—including the original manga, the 1996 OVA, the 2002 reboot manga "Evolution," and the "Decode" anime—Birdy's core role remains consistent, but details diverge. For example, her stance on lethal force varies: some iterations depict her as reluctant to kill, while others show her accepting it as necessary. Her design also adapts, though common elements include distinctive white-and-pink hair and a revealing combat uniform. In "Decode," she operates under the human alias Shion Arita, working as an idol to conceal her identity.

Her development often emphasizes the psychological toll of violence, both witnessed and enacted. The shared existence with Tsutomu challenges her autonomy but fosters mutual reliance, as seen in their efforts to balance his civilian concerns with her missions. This dynamic evolves through compromises and conflicts, particularly when their priorities clash during crises involving extraterrestrial threats.