Birdy Cephon Altera is a bio-engineered Ixioran Altan supersoldier and officer in the Space Federation Police. Her species hails from a planet orbiting Altair and was genetically enhanced for combat. Raised by the android Violin, she underwent rigorous training to become an elite investigator. A pivotal childhood trauma occurred during a terrorist attack on the Central Tower, where Violin sacrificed herself to protect Birdy from a Berserker-type unit. Birdy then destroyed the unit in a berserk state, earning the moniker "Berserker Killer"—a title often misconstrued as indicating general violent tendencies rather than commemorating that specific feat.

Physically, Birdy consistently possesses light blue eyes, long hair blending white and pink, and an athletic build. Her standard uniform is a form-fitting blue or purple leotard with long sleeves, stockings, and sometimes heels. Continuity variations exist: the original manga and OVA depict her hair with the right side white and left side pink, while the reboot manga and "Decode" anime show pink bangs with white elsewhere. She utilizes disguises, including a long black coat and sunglasses, or human clothing like Tsutomu Senkawa's outfits during body-sharing periods. In "Decode," her undercover identity as idol Shion Arita features brown-dyed hair and human attire.

Birdy's personality blends energetic confidence, impulsiveness, and a strong sense of justice, tempered by tactical bluntness and occasional social ineptitude. She demonstrates fierce loyalty to protecting innocents, willing to sacrifice herself. A key divergence across adaptations concerns lethal force: the reboot manga and "Evolution" sequel emphasize her absolute refusal to kill as a core principle, while the OVA and "Decode" depict her pragmatic acceptance of lethal force when necessary. Her moral compass frequently clashes with bureaucratic protocols, casting her as a "cowboy cop" prioritizing ethical action over regulations.

Her abilities derive from Ixioran physiology and Federation technology, granting superhuman strength, speed, agility, durability, and enhanced olfaction for tracking. She can partially or fully transform her appearance for disguises or shifts between her form and Tsutomu's. Her signature "Crash" technique channels concentrated energy through physical contact, often manifesting as an electric shock. The "Q-body" organelle allows transformation into a monstrous, nearly indestructible kaiju-like entity under extreme stress or external command—used sparingly, such as during the Agni lab's destruction.

The core narrative catalyst occurs when Birdy accidentally kills Tsutomu Senkawa pursuing criminal Geega on Earth. To preserve his life, she hosts his consciousness within her body while his physical form regenerates, forcing prolonged cohabitation. Control alternates: Tsutomu maintains his daily life while Birdy handles police operations. Their mental link enables empathetic communication and mutual influence—Tsutomu adopts Birdy's bravery; Birdy gains insights into human vulnerability. This dynamic evolves: in "Decode," Tsutomu increasingly supports Birdy's missions; the reboot manga explores mental connection instability, leading Birdy to enter a two-year stasis to prevent one consciousness overwhelming the other.

Expanded media deepen Birdy's background. "Decode" introduces Nataru Shinmoyu, a childhood friend and fellow Ixioran refugee on Earth harboring unreciprocated romantic feelings. The reboot manga and "Evolution" explore systemic Federation corruption, culminating in Birdy defecting to protect antagonist Christella Revi from execution—rendering her a fugitive. Her past as a potential "breeding slave" due to her genetic lineage surfaces, highlighting political exploitation. These arcs trace her progression from a duty-bound officer to a figure challenging institutional decay, prioritizing individual morality over allegiance.