This page explains how to recruit Leah in Chrono Cross: The Radical Dreamers Edition.
Leah is a cave girl that the party meets at Gaea’s Navel.
How to Recruit
Leah will temporarily join your party when you reach Gaea’s Navel (Home World) as part of your mission to seek the six dragons.
To reach Gaea’s Navel, return to Hydra Marshes (Another World) and follow the west branch to beat up Beeba again. You’ll receive an Ancient Fruit. Go east one screen and step along the narrow branch where another Beeba is, then use the fruit here.
After defeating the Green Dragon at Gaea’s Navel (remember to steal the dragon’s Green Plate!), Leah will officially join your posse.