Responses
there is no love child. the blond is the actual Yoo Hajin, and MC is Baek Hayeon. They have the same parents. Their identities were switched because the blondie was born weak and sick. The Yoo famly only needs one healthy heir so their identities were switched and MC became Yoo Hajin, the first born and only child while the blondie was erased and given their Mother's surname.
Does this mean half bro is older?? Then that means he’s the product of her first love before she got married maybe?? And that he was the real “Hajin” but then because he’s not the grandfathers blood, he wasn’t considered to be a grandchild and therefore when the now Hajin was born between mother and father, he was the only grandchild. And in order to keep up appearances with everyone who knew the first child’s name is Hajin, they swapped the kids names. The blonde dude is the Hajin who lived overseas.
Thank you for listening to my theory… a game theory