Secondary Skills are minor skills used by different party members unless commanded to. For example, Gale, Reverse, and Recover are secondary skills because Zael/leader needs to command the players to use that skill. Most secondary skills are buffs and must be unlocked/gained in order to use them.
List of seconday skills:
Zael: Gale and Power Strike.
Calista: Recover (Heal)
Dagran: Power Strike and Full Counter
Syrenne: Power Chain and Full Counter