It is a special upgrade that does not have the Equipment version. User of this upgrade always does +10% damage to enemies, but his/her health is reduced by 10% as well. Confidence is one of two upgrades in final slots of Calamity, Evelynn, Dex and Jyn. Also, you can get this in the slot machine.
It's not a very good idea to use this for low-health characters, such as Snipers, as it would reduce the already low health. It's recommended for Mercenaries, Juggernauts, and Elites, as they are the most tanky.