In the script editor, the snippet to determine if a hit is a critical is:
CODE
# Critical correction
if rand(100) < 4 * attacker.dex / self.agi
self.damage *= 2
self.critical = true
end
If you want to have a character that has a high change of hitting a critical, make them have high dexterity

If that's not an option, You can go into the script editor, Game_Battler 3, about line 57 has that snippet I pasted above, I've edited mine so it reads:
CODE
# Critical correction
if attacker.is_a?(Game_Actor) && attacker.id == 1
extra_chance_critical = 30
else
extra_chance_critical = 0
end
if rand(100) < 4 * attacker.dex / self.agi + extra_chance_critical
self.damage *= 2
self.critical = true
end
Giving actor #1 (Aluxes) a bonus chance of hitting a critical