Schoolboy Alan on Route 36: Fire Stone
Fisherman Tully on Route 42: Water Stone
Lass Dana on Route 38: Thunder Stone
Picknicker Gina on Route 34: Leaf Stone
They're actually on the main site (http://dynamic.gamespy.com/~psypoke/gsc/crystal.php
Characters #5) but it isn't that obvious so I put it here.