Honesty and the lack thereof.
It says a lot about a character if they'll be truthful even to their detriment, or if they'll lie even when it's no benefit to them. It's not just the "when" of dishonesty, but the "why" of it too. Sometimes white lies really are the kindest course of actions, as dishonest as it may be. On the flip side, there's also the ways they reveal their true emotions and intentions. If you've ever had a friend or relative who said something extremely bigoted out of the blue, you just had a very honest look at the kind of person they are. If you've ever stared down the barrel of the gun, you can rest assured the hand that holding it has all but stopped putting on airs. If you've ever been betrayed by someone you thought you could trust, that act of betrayal was a form of honesty too, in that they showed you their true colors and admitted just how much they've been conniving against you. It's practically a confession. In that way, honesty can be very harmful in the same vein that white lies can be merciful.
But at the same time, in a world with so much deception and cruelty, that makes the sincere acts of kindness and love mean all the more. Clear water is more beautiful when leaving a muddy swamp. The rare characters in such a setting who have such sincere natures, it's both all the more touching seeing them defy the lying world around them, while also being all the more tragic when they have no choice but to follow the same norms themselves.