Jin is really is loyal including Matthew in the end (BIG SPOILER) . . .
. . . .. . . . . . . . Matthew decided to study abroad which made Jin lonely asf and he decided to make a house like he wanted to when Matthew was a child (Jin has no feelings for Matthew when he was a child) now when he was done building the mansion he says "will you live with me forever?" of course they did and I will not spoil any farther