To be honest or sincere is first of all to be intellectually honest with oneself, which is a commitment to what Kant called unity of apperception, whether one thinks of it in such terms or not. As Brandom would remind us, this means to the best of our ability honoring an implicit higher-order commitment — to the consequences of our commitments, and to avoiding incompatible commitments — that we have necessarily made in being committed to anything at all.
We could refer to this as integrity, or a commitment to commitment. Recall Aristotle’s indignation in the Metaphysics against the sophist who refused to honor the principle of noncontradiction.
This is obviously a high standard, if we intend to apply it to people’s emotional responses in ordinary life. We need to be forgiving of the fallibility of others, as well as of ourselves. Honesty to others needs to be tempered with kindness (or wise charity, as Leibniz would say). But we should strive to be integral beings in our emotional responses.
Kindness has no set formula; sometimes something like tough love is appropriate. These things are always matters of judgment. I would go beyond Kant and say we should be kind to all beings, period, but the mode of that kindness should be appropriate to the situation. I am kind to inanimate objects by not engaging in senseless destruction or waste. I may kindly question your conclusion, or tell you what you don’t want to hear. I may even kindly revolt against your oppressive regime. That just means there is no spite or ressentiment in my heart as I take a stand for justice. (See also Intellectual Virtue, Love; Things Said; Interpretive Charity; Affirmation; Genealogy.)