I read this manhwa a while ago and I can't remember the name. It was a girl who protected her sister from a big snake who killed her whole family. She protected her little sister and got eaten by the snake and when she woke up she had pink hair and was in a strange place with people who had magic. After that she killed a lot of monsters and was searching for her sister.
After losing her family in a demon attack, Rosha is reborn as the 777th most powerful magic knight in the Golden Dawn Society. To avenge her family, she trains hard and rises through the ranks against the odds. Then, she is tasked with protecting little Joshua, who is said to be the Child of God...
The main character loses her family to a demonic attack in the first chapter, to "Ramia", a half-serpent, half-woman demon.
She escapes with her sister Lily and tells her to run off and stays behind. God grants Rosha power as she's about to die, and kills the Ramia. She's picked up by a man, is told that there's no trace of her sister, and swears she'll find her sister and kill any other demons she can find.
After the resurrection she trains with and joins a group of demon hunters. She has the explicit goal of trying to track down her sister while hunting monsters.