Rachnera Arachnera, often called Rachnee by those close to her, is an Arachne monster girl who becomes the sixth homestay resident in the household of Kimihito Kurusu. She has the upper body of a mature young woman with short lavender hair, six pupil-less red eyes, and sharp pointed teeth, while her lower body is that of a large black spider. Like others of her tribe, she carries a skull-like pattern on her abdomen, and her overall appearance commonly causes fear in humans who meet her for the first time.

Her difficult background shapes much of who she is. When she was initially placed with a host family through the interspecies exchange program, that family was repelled by her spider-like form and eventually sold her to a man who exploited her silk-producing abilities for profit. Treated as a means of income rather than as a person, she fled and, while on the run, kidnapped Kimihito. This dramatic beginning eventually led to her joining his household, where she became the fifth girl to officially move in after the situation was resolved.

In personality, Rachnera is calm, observant, and highly intelligent, with a sharp wit and a sarcastic streak. She tends to keep her emotions in check and enjoys watching how others react, often teasing or provoking people for her own amusement. She has a strong sadistic or dominant side and takes particular pleasure in using her webbing for binding, a hobby that extends into a serious skill with rope-style restraint. Despite this playful and sometimes intimidating demeanor, she is not cruel by nature. Her initial distrust of humans came from being mistreated and deceived, and she assumed Kimihito's kindness was a pretense; discovering that his compassion was genuine became a turning point for her. Her core motivation is a longing for acceptance and genuine companionship after the rejection and exploitation she endured, and she gradually finds that sense of belonging within the household, where the other residents affectionately call her older sister.

In the story, Rachnera functions both as a romantic interest and as a stabilizing presence among the monster girls. Because she remains somewhat detached and observes situations from a step back, she often acts as an unofficial mediator or adjuster when emotions run high between the more impulsive residents. She is fiercely protective of those she cares about, particularly the younger or more vulnerable monster girls, and is willing to step in forcefully when she believes they are being threatened. Her relationships with the other girls blend rivalry, teasing, and genuine camaraderie, and her evolving bond with Kimihito moves from captor and captive to mutual trust and affectionate partnership, with her referring to him affectionately as honey.

Rachnera's development is marked by a gradual opening up to others. Her initial cynicism toward humans softens through her experiences in the Kurusu household, and moments of crisis reveal a protective and caring side beneath her cool exterior. She learns to trust and to accept care from others, while still maintaining her independent and composed nature.

As an Arachne, Rachnera possesses notable physical abilities. She can produce webs of exceptional strength and elasticity, which she uses for capture, defense, construction, and even crafting clothing and hammocks, and she is a skilled weaver overall. Her spider body grants her superior agility, enhanced strength, and the ability to climb surfaces easily, and she has venomous fangs that can inject poison. Her strategic and analytical mind makes her an effective problem solver and a dangerous opponent when provoked. She can breathe through her human mouth, with additional breathing organs on her spider abdomen, and because she dislikes soaking in hot water, she prefers showers over baths. Taken together, these traits make her a capable, layered figure whose intimidating appearance contrasts with the loyalty and warmth she shows to the people she comes to trust.