Anthea Kallenberg is the central protagonist of the story and a member of a covert ops unit known as Flowers, designated as Flowers No. 03. She is a seventeen-year-old girl of Scandinavian origin, specifically Swedish. Her existence is widely known and feared by military personnel around the world due to her exceptional abilities.

As a child, Anthea was personally raised by Colonel Amagata, which forged a strong connection to the military hierarchy that controls her unit. She takes considerable pride in her combat prowess, a reputation that is well-earned given her consistent performance in the field. Her skills make her the logical choice to take the lead on special operations, and she is frequently deployed to extract other agents from dangerous situations. Her primary areas of expertise include infiltration and assassination, though her abilities are not limited to stealth, as she possesses high-level combat skills.

Anthea is driven by a deep personal need to discover her true identity. This quest for self-understanding is a core aspect of her character, as she navigates a life defined by forced service. Her role in the story often places her in direct confrontation with other super-powered individuals. In a significant mission, she is deployed to capture a target named Shunichi. During the ensuing battle, she demonstrates not only her offensive capabilities but also a unique power set that includes generating a defensive shield and a remarkable ability to regenerate her own skin and tissue after sustaining severe damage. This regenerative ability, however, is not without limits, as she experiences extreme agony when her shield is overpowered. In the climax of this confrontation, she employs a power negation ability to counter an enemy's shield before ultimately completing her mission by shooting her target, acknowledging a shared error in their circumstances with the words, "Hana, we were wrong again". Her appearance is notable for the Flowers pendant she wears attached to her pigtails via two hair clips.