Years ago, during the war between the Three Factions, all sides suffered heavy losses. They tried to recruit humans to their cause. Most humans were cannon fodder, but the strongest, like the Hero Clan, were forced in.
Basara's father, Jin Toujou, was a genius of the Hero Clan with Devil King-class power. He ended up in a chaotic battle against Sephiether, the sister of Wilbert (the former Demon Lord). Raphaeline, a God-rank entity, was sent by Heaven to suppress them both.
However, due to the world's rules, she couldn't use her full power. Jin and Sephiether both went berserk, and Jin's spatial powers tore a hole in reality. The three drifted through the void and ended up in the lair of a sleeping Ancient Dragon.
To return home, they had to cooperate. They didn't fall in love; Raphaeline actually preferred women—she and Chisato were lovers, though they claimed to be "sisters." The Ancient Dragon took an interest in Jin because of a faint dragon lineage in his blood. With the dragon's help, Jin's bloodline was purified, but he couldn't handle the influx of power. Like all dragons, he needed to vent that energy sexually.
Sephiether and Raphaeline refused, but the Dragon wouldn't let them leave. Finally, they reached a deal. Sephiether used a secret method to draw the excess essence out of Jin, intending to store it. Raphaeline signed a contract to protect them. But Sephiether, only being Devil King-class, couldn't control the essence; it fused with her own blood and formed an embryo.
Sephiether couldn't accept being pregnant from an accident. As she was about to destroy her own life, Raphaeline—desperate to return home—offered to act as a surrogate. Raphaeline took the embryo, which absorbed her own goddess bloodline, making it even stronger.
Basara Toujou was born of compromise, not love.
As a God-rank being, Raphaeline could choose to conceive, but it weakened her significantly. When Basara was born, she was at her most vulnerable. The Ancient Dragon finally opened a portal and let them go. Jin raised Basara, and Sephiether went into seclusion to break through her limits. Jin became God-rank with the dragon's help.
The only loser was Raphaeline. Even though she remained a virgin through her surrogate method, her bloodline was depleted. The other Chief Gods of Heaven found out and sealed her as punishment.
Chisato (Aphrodite) watched her sister/lover be sealed and couldn't stop it. She wanted revenge against the other Chief Gods. She began looking for someone with the potential to help her. Her first choice was Basara, but his progress was too slow.
Then she saw Ethan. He was progressing at a terrifying speed and had the Boosted Gear. She had been watching him since his first day at Kuoh Academy. She discovered his secret of dual-cultivation and his transmigration. She shifted her focus entirely to him.
She chose to be proactive tonight because she was betting everything on him. After observing him for over a year, she knew no one had his potential. She also realized that, despite his chaotic love life, he was trustworthy.
And finally, there was her own desire. After spying on Ethan's "bedroom activities" for a year, the Goddess of Lust and Beauty couldn't hold back any longer. She had been starved for intimacy since Raphaeline was sealed.
By sleeping with Ethan and signing the Soul Contract, she had tied their fates together. If Ethan reaches God-rank like Jin Toujou, he won't have to fear the Ten Chief Gods, but they will certainly come for Chisato once they realize what she's done.