They are twins, lol. His younger twin went to this world to get the "Cure-All" to save his mother and help his brother, but he was betrayed, which is why his older brother, the MC, is now fighting as a masked OP guy called Cain, to avenge his brother that went on a hard adventure for five years to save his mother and failed, because he was backstabbed by his group.
What was truly tragic was that their mother died two years before he was betrayed.
I know right, that is even worse because he was too late anyway.
They look like they are twins