This is everything you need to know about how to recruit Harle in Chrono Cross: The Radical Dreamers Edition.
How to Recruit Harle
Harle is one of the strongest characters you can use but, sadly, there’s no way to permanently recruit her. She can only temporarily join your party. During your first playthrough, anyway.
When Serge switches bodies with Lynx and enters the Dimensional Vortex, Harle will become your guide and automatically be added to your party. After you rescue Riddel, she will leave again.