Heeseung is your brother that has leukemia (which he got it from your dad). He fought co condition for 4 years alone.. Your father left your mother because of his 'secret'.. Later on, Your mother fpund another guy and got married and that's when your second old brother named "Park Jay" was made... But Jay's father died from stroke. So your mother and Heeseung's father got back and that's when you (Lee Seona) got made... You always feel so sick and always absent in school because of it..