your sister got into serious trouble with the monster, she steal something from the monster, and to return, she has to marry a monster

your family love your sister much more than you, so they forced you to marry the monster, replacing your sister

you married the monster, the monster can turn into a human form, but he always think he's a monster even in a human form. he always avoid you, he afraid you scared and he thinks you don't deserve a monster like him

you never even talk to him