Falfa is a slime spirit and one of the main characters in the story. She, along with her twin sister Shalsha, was born from the conglomerated spirits of the countless slimes that the witch Azusa Aizawa killed over her three hundred years of life. Despite being the result of these slain slimes’ lingering resentment, Falfa manifested as a cheerful and affectionate being who immediately bonded with Azusa.

In appearance, Falfa looks like a young girl of approximately ten years old, though her actual age is about fifty. She has sky-blue hair, usually styled into twin tails, and green eyes. Her personality stands in stark contrast to that of her sister Shalsha, as she is portrayed as energetic, outgoing, playful, and openly expressive with her emotions. While Shalsha was initially driven by a desire for revenge, Falfa never shared that animosity. Because she owed her very existence to Azusa's actions, she felt a familial bond with the witch rather than a grudge, and she appears at Azusa’s home without invitation to call her "Mama" and to warn her of her sister’s impending arrival. After Shalsha’s defeat, Falfa is instrumental in persuading her sister to abandon the cycle of vengeance, arguing that it would be impossible to hate every person in the world who has ever killed a slime. Following this, both twins are welcomed into Azusa’s household, where they become beloved members of the family.

Falfa’s primary role in the story is as one of Azusa's adopted daughters and a source of warmth and levity within the growing household. She is deeply attached to her new mother and is often seen seeking her affection, playing around the house, or expressing excitement over simple pleasures, such as a good meal being prepared by their dragon companion, Laika. Her key relationships are with her immediate family. She loves her sister Shalsha dearly, though the quieter and more mature Shalsha often has to take the lead when Falfa becomes frightened. She also acts as a caring elder sister to Sandra, another child later taken in by Azusa, even helping to teach her how to read and write.

Despite her childlike demeanor, Falfa possesses notable abilities and an exceptionally sharp intellect. While she cannot use magic, she is a genius in practical sciences and mathematics. Her proficiency is so advanced that she has even published academic papers that are highly regarded among scholars, and during a visit to a school, her knowledge surpassed that of the teacher, causing the instructor distress. This intelligence is paired with a practical and sometimes blunt mindset, as evidenced by her occasionally straightforward solutions to problems. A unique aspect of her being is her origin as a slime spirit. On one occasion, after getting a stiff neck, she reverted to her original form as a small, dark blue slime. In this state, she lost most of her conscious reasoning and behaved like an ordinary slime, eating random things off the ground, until her family found a way to restore her humanoid shape.