far as I can tell, devoted strike is basically a variant of the basic attack that changes it to boost your tp-power more when you attack with it compared to the regular attack, while also scaling off of both your usual attack modifier and your devotion value. To explain that better, devotion makes attacks with green icons (like devoted strike) better in general, while devoted strike in particular has damage scaling off of both devotion and whatever the attack stat you have is (so if you give it to valorie, it'll be endurance + devotion, I think?).