Confidence is a Thief bonus that increases and decreases based on the success and failure of Thief-related activities. It represents your self-confidence in your own abilities. Backstabbing and Thief ambushing has the most noticeable effect on gain, whereas dying and being arrested have the most notable effect on loss.
Confidence drifts to a neutral state over time, and the base level for this neutral state is "average confidence." High Charisma will increase the base level of this neutral state, and will also adjust the speed of the drift to your benefit. I.e. high confidence will drift more slowly and low confidence will drift more quickly back to your neutral state. Charisma also places a cap on the highest level of confidence you can reach.
|Lvl||Message||Min CHA||CHA for baseline|
|-5||You attempt to smirk to yourself, but don't see how things could get any worse.||-||-|
|-4||You smirk to yourself, but you know you aren't fooling anyone.||-||-|
|-3||You smirk to yourself, but cannot shake the thoughts of your recent screw-ups.||-||-|
|-2||You smirk to yourself, but have to force it.||-||-|
|-1||You smirk to yourself, but feel slightly uncomfortable.||-||-|
|0||You smirk to yourself, but can only muster up average confidence.||-||-|
|+1||You smirk to yourself, feeling a little above average.||10||50|
|+2||You smirk to yourself, feeling rather confident about things.||15||60|
|+3||You smirk to yourself, feeling quite good about how you're doing.||20||70?|
|+4||You smirk to yourself, the way only a stylish Thief can.||20||80?|
|+5||You smirk to yourself, knowing you are at the absolute top of your game.||30||90?|
EDIT: 5/8/2018 (changed +4 bonus minimum charisma. I currently have that level of confidence with only 20 charisma) "B"