To unlock the hidden characters, you must have the appointed amount of gold coins to trade with them.

Basch (FF): require 30 gold coins
Yuffie (FF): require 30 gold coins
Cloud (FF): require 30 gold coins
Sephiroth (FF): require 30 gold coins
Maribel (DQ): require 30 gold coins
Marcello (DQ): require 30 gold coins
Kiefer (DQ): require 30 gold coins
Trode (DQ): require 30 gold coins