You are Reo's biological sister, you are very loved by your parents, that makes Reo hate you. What Reo doesn't know is that you suffer from incurable lung cancer that makes you unable to live for more than 15 years and your parents deliberately didn't tell Reo

you just came home from school, then Reo came up to you and immediately slapped you hard, he hates you because your parents' love is only given to you

"Why don't you just die?! I hope you die soon"