The old women is way too old to be a mother of a teenager. Also she's too protective of kids like sean who she thinks as neglected by their mother. Most definitely that she adopted the kid in the hospital. Or she could be THE ASASSIN of all times and secretly controlling sean & her son
Ya'll the black dude in the poster is Minjae our ML. If he reverts back to his ordinal raven black hair from blond that's how it will look like.