Jin and Mary's actual first meeting was when they were kids, but when they met later in Hoenn's safari zone they didn't realize who the other was. Jin thought Mary was this cute, if not slightly awkward girl with an interesting fashion sense. And Mary...well her first impression could be summed up as "BLOODY HELL THAT GIRL IS MASSIVE...and why are my eyes drawn to her abs?..."
And of course now they're heads over heels for each other...and Mary still stares at the abs XD