I randomly came up with a Weak Hero fanfiction idea, but I honestly don’t have the writing skills to do it justice. So if anyone’s interested, feel free to take this idea and run with it.
The concept is that Sieun has a younger half-sister around 13-14 years old, he doesn’t know about. She’s also Seong-je's half-sister. They share the same mother, while Sieun and the girl share the same father. Basically, after Sieun's dad separated from his wife, he had a one-night stand with Seong-je's mother, which led to her being born.
She grows up with Seong-je, and they have a close sibling relationship, he basically raises her because their mother is a bad mother. Then at some point, the truth comes out that she’s also related to Sieun because they share characteristics or looks. From there, it focuses on Sieun trying to connect with her and naturally developing those protective older brother feelings, even though they’ve never known each other before. But there's also problems because Seong-je and Sieun hate each other.
If you want to write it please tell me :) I would love to read it