To be forgiven for sin is an act of the mercy of God. And forgiveness requires repentance, which can be explicit or implicit. So it might seem as if repentance were always required to receive mercy. But forgiveness for sin is not the only way that we receive mercy.
Every grace received by us, every grace offered to us, is an act of the mercy of God. If we sinfully reject an offered grace, the offer by God was still an act of His mercy. For God continues to be merciful and loving, even when we choose to sin. Our acts are sinful because we rejected the grace of God to avoid sin.
Fine. But when we reject grace, we are rejecting mercy. So how can we receive mercy without repenting? The answer is prevenient grace. It is the grace of God given to us prior to any possible cooperation on our part. Every sin is a sin because we were given the prevenient grace to understand good from evil and to be able to choose to do good and to avoid doing evil. Every sinner, even the worst hardened unrepentant sinner, certainly necessarily received prevenient grace — otherwise he would not be guilty of sin, since he would not have been given the first graces needed to recognize and avoid sin.
See my post: Prevenient Grace and Subsequent Grace
Prevenient grace is God operating, in His great mercy, to give us grace regardless of our holiness or sinfulness, with no possibility of cooperation. It is God operating, not cooperating. Subsequent grace occurs only after the reception of prevenient (or first) grace. Only then can we possibly choose to cooperate with grace, or not.
And the prevenient graces given to all of us is a reception of the mercy of God.
Forgiveness requires repentance, because God decided to give us the great gifts of reason and free will. These gifts make us true persons, like the Persons of the Trinity. But these gifts also give us moral responsibility, including the responsibility, after reception of prevenient grace, to respond to the offer of subsequent grace.
Please take a look at this list of my books and booklets, and see if any topic interests you.