Zest is a Demon who originally served her creator, the Demon marquis Zolgear, as his aide.

When a pair of sexy girls, Maria and Mio, come to live with high school student Basara Toujou, his life changes forever. The two girls have a secret that they don’t keep for long—Maria is a succubus, and Mio is a future Demon Lord. But Basara has a secret of his own—he’s the last descendant from a clan of demon-fighting heroes. To make things even more complicated, the girls force Basara into a master-servant contract, and Basara ends up becoming their master! What happens when you put a succubus, a demoness, and a demon-fighting hero under the same roof? They form a bond that will help them to take on anything! 
 
(Source: Funimation)

A Japanese voice actress affiliated with Mausu Promotion.

The second season of Shinmai Maou no Testament. 
 
After two demon girls, Maria the succubus and Mio the demoness, force Basara into a contract that makes him their master, the trio form a complicated master-servant relationship. Basara is the last descendant of a demon-fighting hero clan, but as master of two demon girls, he finds himself defending them instead of fighting them. 
 
(Source: Funimation)

The cast takes a vacation after resolving the conflict in the demon world.

Specials included on the Blu-ray/DVD volumes. Super deformed versions of Mio and Maria narrate Basara's various erotic encounters

When his oddball father suddenly decides to remarry, high school student Toujo Basara is about to acquire some very peculiar new family members. Enter his new seductive step-sisters, Mio and Maria. When Basara discovers that one of his step-sisters is a succubus and his other a recently crowned demon lord, all hell breaks loose!
 
Basara is almost forced into a contract with his demonic sisters where he would act as their eternal servant—but something goes wrong. Instead, a reverse contract is formed, and Basara becomes the master. Between fending off their seductive advances to coping with the attacks of rival demon contractors, Basara has a big learning curve if he is to survive.
 (Source: Seven Seas Entertainment)

A special included on the Blu-ray/DVD volume. Super deformed versions of Mio and Maria narrate Basara's various erotic encounters.

High school student Toujo Basara’s life has taken a turn for the strange. Although hesitant about his father’s sudden decision to remarry, he can’t deny it comes with a couple perks—drop-dead gorgeous step-sisters…who also happen to be members of a demon clan! Mio Naruse is a Demon Lord and Maria Naruse her succubus servant. Chaos and passionate antics ensue as Basara gets acquainted with his new demonic siblings, trying to protect them from dangerous demon rivals and struggling to keep up with their lusty, devilish appetites.
 (Source: Seven Seas Entertainment)