Jin's personality is actually pretty consistent from when she was younger, energetic and outgoing. (tho there was a point in time where she was in a much darker place) but the main difference for her now is that she knows when a situation calls for her to get serious.

When Mary was a kid she was considered a bit...odd by her parents, or at the very least she wasn't always the prim and proper lady her family expected her to be. After the attack that took her parents she of course was in a VERY dark place for a long time. But after time with her aunt and uncle she was able to get back some of that joyful spark from her childhood.