Henrietta is one of the first generation of cyborg assassins working for the Social Welfare Agency in modern-day Italy. Before being brought into the program, she was the sole survivor of a brutal home invasion in which her family was murdered. During this traumatic event, she was also assaulted and left for dead, suffering severe psychological damage that made her suicidal. Roberto, her future handler, selected her for the Agency's secret program, giving her a new chance at life through cybernetic enhancement and intensive conditioning, a process that also suppressed her memories of the past.

In her daily life, Henrietta appears as a polite, soft-spoken, and seemingly innocent girl with short brown hair and brown eyes. She comes across as well-mannered, a remnant of her upbringing in a wealthy family before her tragedy. However, this gentle exterior masks a personality profoundly shaped by the conditioning she underwent. Her entire sense of purpose and emotional world revolves around her handler, Jose Croce. She harbors strong, childlike feelings for him that go beyond simple loyalty, often displaying visible jealousy when she perceives he is paying attention to other women, including the other cyborg girls. Her devotion is absolute, and her primary motivation is to please him, a drive that she measures in part by the number of enemies she eliminates on his behalf. Jose, for his part, tries to give her a semblance of a normal life, sharing his interest in astronomy with her and giving her gifts such as a diary and a red coat. He also encourages her to play the violin, which helps her gain control over her powerful cyborg body.

In the context of the story, Henrietta serves as the primary lens through which the complex and tragic relationship between a cyborg girl and her handler is explored. Her close bond with Jose is contrasted with the colder, more professional relationships of other pairs, highlighting the varied ways the Agency's conditioning manifests. Her key relationships include a sisterly bond with another cyborg named Triela, who often acts as a supportive older figure for her. Henrietta also looks up to Jose's colleague, Jean, but her world is ultimately centered on her fratello, or "brother".

Henrietta's development throughout the series is marked by a tragic decline. The conditioning that gives her purpose is slowly breaking down her mind. She begins to forget things she has written in her diary, a source of great distress, and her emotional state becomes more fragile. Her devotion to Jose intensifies to a point where it becomes a dangerous obsession, with hints that she would not be able to survive a world in which he did not love her. After a mission triggers traumatic memories of her past assault, she suffers a mental breakdown and is subjected to a harsh reconditioning process that wipes away many of her memories and leaves her emotionally blank, a shadow of her former self. Despite this, her tragic story ultimately concludes in a final, mutual act of destruction with Jose, whom she both loves and cannot live without.

As a first-generation cyborg, Henrietta is equipped with augmented strength and reflexes, allowing her to perform incredible physical feats. Her preferred weapons are the Fabrique Nationale P90 submachine gun, which she famously conceals inside a violin case, and the SIG Sauer P239 pistol. She and Jose have also been known to use the Walther WA 2000 sniper rifle for long-range assignments. Despite her small stature, she is a highly effective and dangerous operative, though her effectiveness is often tied directly to her emotional state and her desire to be praised by her handler.