Astraea, designated Type Delta (Δ), is a first-generation Angeloid engineered by Synapse for close-quarters combat. She possesses long blonde hair, red eyes, stands 159 cm tall, weighs 48 kg, and has measurements of 91-56-86 cm, featuring the series' notably large bust. Her attire is typically blue, matching other Angeloids, and she bears non-variable wings optimized for hyper acceleration.

Intellectually, Astraea exhibits significant deficiencies, frequently lacking processing capacity and common sense. She struggles with basic survival tasks, once attempting to bury herself for nutrients. This stands in stark contrast to her formidable close-combat abilities. Her design prioritized high combat strength and emotional capacity over computational intelligence, resulting in clumsiness during non-combat movement.

Sent to Earth to eliminate Tomoki Sakurai and retrieve rogue Angeloids Ikaros and Nymph, Astraea initially clashes with Tomoki. Her mission consistently fails due to distractibility and developing emotional connections. During the Chaos Arc, ordered to kill Nymph, she hesitates. Encouraged by Tomoki to follow her own desires, she shatters the chain binding her to Synapse, liberating herself from external control.

Her combat capabilities include the Aegis-L Shield for frontal defense and the Chrysaor photon blade, capable of slicing through any Angeloid shield. Her acceleration-type wings enable increasing speed during prolonged engagements, though she remains vulnerable to long-range attacks. She demonstrates these abilities fighting Chaos, whom she perceives as a younger sister, shielding Chaos with the Aegis-L and attacking with Chrysaor. Chaos exploits Astraea's close-combat limitations to gain an advantage.

Emotionally, Astraea develops strong bonds, implying romantic feelings for Tomoki and exhibiting a protective sisterly dynamic towards Chaos. Her empathy runs deeper than other Angeloids', leading her to openly express concern, joy, and frustration. This growth fuels her defiance of Synapse for Tomoki's group. She later participates in collective efforts, like confronting the clockwork Angeloid Hiyori Kazane in the movie, assisting Ikaros and Nymph in battle and containment efforts during Hiyori's self-destruct sequence, though her role remains secondary.

Following her liberation, Astraea resides with Tomoki's group, engaging in daily misadventures and becoming a frequent target of Tomoki's schemes. Her integration includes playful scuffles with Nymph over resources like food. Despite this, she consistently aligns with the group during critical threats, solidifying her shift from antagonist to ally.