Here's some facts and other things you may not know because I know some of you are or might be confused.• The Holy War took place 3000 years ago. Between the Demon Clan and Stigma Alliance (Goddess Clan, Fairy Clan, and Giant Clan.)
Chastity is in the middle of all of this absurdity because her father comes from the Demon Clan & her mother came from the Goddess Clan. She was born in efforts to prevent the war from happening but, obviously that didn't work out.
She was born with an innumerable amount of magical power (including the power the demon king placed in her). Her parents were slain by the Stigma and she was raised by the Goddess Clan with Elizabeth growing up alongside her.
The Goddess Clan treated her horribly (Ludociel would torture her, ripping her wings off), sometimes she would be pampered by Sariel and Tarmiel who felt bad about her predicament. It wasn't her fault she had this power, instead of killing her as a child they decided to raise her to become a weapon of mass destruction.
They would send her out on missions to display her power and gave her an option of weaponry when going on missions. The weapon she chose was a scythe. Which is why they are would call her "the reaper."
Instead of going on missions, at the age of 16 she stopped, often going to clear her mind in a meadow. This is where she meets Zeldris, who had also traveled there to clear his mind as well. Once his father learned of their meet ups (because obviously Chastity spoke to Zeldris and became his friend) he would order Zeldris to get closer to her as a test to see if he could gain her trust.
Chastity became lovers with Zeldris as they both truly dreamed of leaving before the 'Holy War' took place. She soon became friends with Meliodas who had stumbled across the girl when searching for his brother.
Meliodas took a liking to Chastity as well which resulted in Zeldris and Meliodas arguing over her. Estarossa wouldn't meddle in that but he would often watch the girl (like the stalker he is, what a creep).
Once the Demon King learned of her potential he knew he had to seal her away, which he did in battle.
Remember Chastity doesn't fully remember everything. She has about 70% of her memory.
Addition: Gelda's story won't be the same for the sake of the plot of this story.