There is no stat to boost crit power. Str will boost your patk and thus the damage you do (normal and crits) but there is no crit power "stat" which is why you dont see it in the window. The closet you get is the crit power passive boost a few classes get that makes them hit harder on crits.
Well Crit powers are not secret, but I agree it would be nice to have a stad showing the crit power rate.
All I know is that when my Glad wacks with his duals he usually gives like 200 damage/swing, with crits he gives like 800-850 damage (if I remember right)
Thats when he is fully PP/SE buffed.
In your character window where all the active skills are, switch to your passives and it should be there. And also unless you are buffed with death whisper have the passive skills or crt dmg on your weapon your crits will do x2 normal dmg.
This stat like shield blocking rate is hidden. I can't tell why. Maybe it's a big secret. We just know "the bigger, the better". Of course, even if we knew it, there are not many things we can understand about it. There is a chance though, that with careful observations someone could have some interesting results. For example "for x amount crit. power I do y amount the normal damage". So it wouldn't hurt to be visible.