Crosselle is one of the 4 Demon Gods who ruled over Demon World since the very beginning. In the distant past, Lilith and Erina defeated all the Demon Gods after a long battle. However, the souls of the Demon Gods weren't completely eliminated, and they started seeking ways to be reborn.
The sly strategist Crosselle chose to form contracts with Demons. Finding an adequate host, Crosselle tempted Monica, the head researcher of 'Cyber Robotics', a company in charge of Demon World's android production, with a promise to free her from all pain. Monica, who was suffering from endless tasks and fatigue, accepted Crosselle's offer. Crosselle possessed Monica's body as planned. The moment Crosselle's plan of slowly possessing Monica's body and regaining her strength was foiled was when she was discovered by Lilith and Erina. Lilith, who bound all 4 Demon Gods with a special binding artifact, spoke of the 'time of ruin' from the prophecy. She promised to give the Demon Gods freedom in the future, and suggested a contract of obedience to the Demon Gods and the hosts. Bellial abandoned the host and escaped, but Crosselle, Andras, and Pymon all accepted Lilith's offer and followed her orders under the offer of freedom.
At the time of the contact, Monica made one condition. Speak and act with class. Crosselle didn't think too much of this condition, but this was a grave mistake. The contract with Monica was the greatest suffering she ever experienced. Any time Crosselle attempted to act like a Demon God, she had to deal with Monica's unbelievable nagging. She couldn't laugh creepily. If her nail art got damaged during battle, Crosselle would face continuous scolding that would make her lose her wits. Monica was most upset when Crosselle ruined her new luxury coat. A skull-cracking scream continued non-stop. Crosselle couldn't endure it any further. The screams stopped after Crosselle cooperated with Lilith, and worked for three days and four nights to buy a new coat. Through this process, Monica achieved complete surrender from the Demon God Crosselle. This feat was historic. No other being was able to achieve this, not even the gods from heaven nor the hero of legend. Crosselle now lives a 'disciplined life' which perfectly follows Monica's rules.
Before she knew it, enjoying a glass of sparkling barley tea after returning home from a mission became Crosselle's only pleasure. Worn down to the bone by Lilith's everyday orders and Monica's constant nagging inside her head, the taste of sparkling barley tea that Crosselle got to taste by chance was so fantastic that it was enough for her to forget being reborn as a Demon God. Nonetheless, as Monica only allows one glass a month saying she needs to watch her weight, the 'Sparkling Barley Tea Day' that comes once a month is most precious to her. For Crosselle, sparkling barley tea is the only thing that motivates her to endure till the day she becomes free in the distant future.