This page explains how to recruit Zappa in Chrono Cross: The Radical Dreamers Edition.
Zappa is a skilled weaponsmith from Termina; he’s the father of Karsh of the Acacia Dragoons.
How to Recruit
You can recruit Zappa after reaching Termina in Home World. Enter the smithy in the nothern district, which is one screen right and one screen up from the town entrance. Zappa and his wife will be standing by the far corner.
Approach the pair and you’ll learn they’re closing the smithy. If you brought Radius along, Zappa will say “hi” to him. In any case, Zappa decides to join you to search for Rainbow material.