If I remember correctly, he wasn't so kind in the game either... or was it later? I remember his worst "brat" moment was in the Deo Path though...
Interesting discussion about amnesia.
Just a correction though : personnality is not innate, nature is.
If Luke grew up in a different environment after his amnesia, it's normal he would get a different personnality. And he did, in a way : he was confined to the manor, everyone was always looking at him hoping he would get his memories back quickly etc
Anyway, about the pinky swearing, I think it was mentionned in a skit that he didn't like it because his father never used to keep his promises or something like that? That memory went away seven years ago, so the Luke now has no reason to hate pinky swearing.
Anyway, one thing I noticed about Luke and Asch while taking screeshots...
Might be spoilerific, but they probably intended for us to notice it and speculate :