Carmilla Vanstein persistently secured her role as the loyal maid to Lindabrea Fafnir Gildmerag (Hanako), following her when Hanako departed her noble household to explore the world, including locations like Edgardo. During these travels, Carmilla attacked and fed on the blood of locals to survive. After Hanako began working at Genzo Shibata's pet shop, Carmilla took up residence there, living rent-free without contributing labor or financial support.

Physically, Carmilla features long light ash blond hair, red eyes, pointed ears, and extendable fangs. Her attire typically consists of a purple Victorian-style sleeveless dress with a high-low skirt, a black star-shaped collar, fingerless puffy long gloves, black stockings, and black kilties. She also wears black pants featuring a small grey bow and sports two ear piercings on her left ear, with red-painted nails.

A Lesser Vampire of the lowest caste, Carmilla is significantly weaker than Elder Vampires or Great Ancestors but retains physical strength surpassing average humans, beast-men, or orcs. Her abilities include enhanced strength, retractable foot-long claws, regeneration through burial in impure soil after injury or death, proficiency with a double-sided great axe, blood consumption for sustenance (with a preference for maidens' or virgins' blood), transformation into purple mist to pass through solid structures, seduction due to her attractiveness, and cursing using the dark powers of her Evil God Zenarith. Her common vampire weaknesses include vulnerability to holy water, sunlight, and garlic, requiring an umbrella for daylight navigation.

Carmilla exhibits a lazy, rude, and arrogant personality, frequently engaging in alcoholic binges that lead to blackout drunkenness. She displays NEET tendencies, often resorting to violence or impractical solutions to resolve conflicts. Spiteful and grudge-holding, especially against Genzo due to their frequent physical altercations, she schemes or curses him in retaliation. Carmilla freeloads off others, living rent-free at the pet shop while refusing to work for anyone except Hanako. She spends money wastefully on expensive alcohol and extorts subordinates to fulfill her desires.

Her most defining trait is an obsessive devotion to Hanako, whom she idolizes and clings to excessively. Carmilla constantly seeks Hanako's approval through flattery and physical closeness, such as crawling over her clothes. This obsession manifests as hypersensitivity; perceived coldness or criticism from Hanako triggers intense sadness or emotional outbursts. Hanako views Carmilla as incompetent and useless.

Carmilla underwent notable development through wrestling training under Genzo's tutelage. Initially dismissive of him, she sought his guidance to defeat Rose, a higher-class vampire servant of Princess Joanna. This training honed her combat skills, enabling her to perform advanced techniques like the Frankensteiner. During a wrestling event, Carmilla initially gained the upper hand against Rose by applying Genzo's teachings but lost due to premature celebration and distraction. Following her defeat, Hanako consoled Carmilla, declared her the "real winner," and offered food—a rare gesture of validation that moved Carmilla to tears of joy.

Carmilla habitually denies her vampiric nature, fabricating excuses to justify her abilities and weaknesses. She sustains herself in the human world by negotiating monthly blood provisions from Genzo.