Created as the ideal aide for demon marquis Zolgear, Zest possessed a tall, shapely physique with white hair, tanned skin, and golden or greenish eyes, complemented by distinctive rabbit-like horns and tail hinting at possible rabbit remains origin. Serving as Zolgear's loyal right hand, she executed tasks like scouting the Toujou Residence with Lars to monitor Mio Naruse. Despite her loyalty, she maintained independence, advising against impulsive actions such as preventing Zolgear from raping Mio to preserve her powers.

Zest lost her original purpose after Zolgear's defeat, aligning with the Moderate Demon Faction under Sheera. During this period, her physical appearance altered, including a significant increase in bust size. Sheera facilitated a master-servant contract between her and Basara Toujou to bolster their strength and secure her protection amidst escalating factional tensions. Though the contract formed correctly, her persistent self-doubt about worthiness triggered its aphrodisiac effects, requiring Basara's touch for relief.

Her personality evolved from cold and calculating, revealing an underlying meekness and timidity outside combat. She frequently adopted a maid role, addressing Basara with "-sama," while wrestling with deep-seated insecurities about her usefulness. Genuine emotions and loyalty toward Basara developed over time, culminating in her becoming the first in his group to conceive his child. This followed her integration into the household, where she assisted domestic duties like helping Mio Naruse improve cooking skills to prevent accidents.

In the *Departures* OVA, she participated in group activities including a waterpark visit and wedding-dress roleplay. Her combat role diminished as she focused increasingly on domestic support within the Toujou household.