FamilyTeam
This strength serves more than me alone.
Each weapon has its own percentage, and that can be affected depending on your character's luck level and whoever that person is paired up with, I believe.Criticals are also in FF7 and probably FE (don't know because haven't played) but I don't remember the percentage or requirement for that happening.
Atleast that's how I think it works in Awakening.
I have no idea how much additional damage it does... but I know I very rarely see anyone surviving a crit.