Zest is the former secondary antagonist of the first season of The Testament of Sister New Devil and acts as an enforcer to her creator/father Zolgear.
At some point she is created by Zolgear as an enforcer and is created in a way that she'd lose all her powers the moment she lost her virginity and she was sent by the current Demon Lord faction to act as scout alongside Lars to survey the conditions of the Tojo Residence she witnessed the subjugation of Mio and Yuki by Basara which made her, for the first time, imply that she has the capacity to feel and wonders if Zolgear could love her as Basara did to his servants or if Basara were her master instead, but later disregarded this idea as a fantasy and Zolgear has an ultimate plan as she orchestrated the plan to kidnap Mio, unbeknownst to the other members of the Current Demon Lord Faction eventually a Puppet Copy of Lars infiltrated the mansion with Basara unconscious and confronted Zest who at the time had no idea that the Lars she met is a decoy and the decoy mentioned his purpose was to drop off the defeated Basara, which was also an object of Zolgear's interest as he was able to control the sword Brunhilde. the decoy asked her whether Zolgear was responsible for the kidnapping of Mio, which was deemed illegal as Zolgear did not let anyone from the Current Demon Lord faction to learn that fact. Zest denied this as Lars's decoy pressed her to allow her to search the mansion, during which Zest deemed Lars as a threat to her master and killed the decoy without knowing that the real Lars is not killed then Basara was struggling against a deluded Maria, Zest came into conflict with Yuki who managed her way in by herself and Yuki freed Mio from her chains and the combined effort of Yuki and Mio proved too much for Zest and she was tied and was originally supposed to act as a hostage for the Moderate Faction, during which she may be able to spill the Current Demon Lord's secret. She, however, resorted to silence as Mio and Yuki planned their next move however Zolgear returns from his trip and was not pleased by the fact that Mio was freed from her chains and that he lusted to have Yuukiin his colection and deemed Zest as incompetent and was set to kill her, only to have his spell interrupted by Basara who has just sorted things out with Maria.
After Zolgear's defeated later by Mio's powers, she was in a state of shock, losing her original purpose as Zolgear's aide, she gave in to Basara's proposal to temporarily move into the Tojo Household along with Sheera (which was revealed to be Maria's mother instead of the alleged 'sister') until the Moderate Faction pick them up. She'd still address Basara with his full name as the members of the household is constantly vigilant against her. Lars, who supposed to be dead, was alive and well, and later finished the affairs with Zolgear who managed to escape the fight after being injured by Mio's powers, as Lars ultimately killed him, effectively ending the life of Zest's first master.
Later, she left the household with Sheera to Wilbert Castle. Before she left, Basara told her that she'd always be welcomed if she wanted to return to the house, in contrast of how the other girls think.