A repentant person is sorry for something wrong they have done, and it shows. We use the word for people, and also for looks and voices: a repentant smile, a repentant letter. It often has a religious colour, like repent, but you can use it in ordinary stories too — the repentant child who comes back with the broken cup. Though actually, it is still a rather bookish word. In speech, people are more likely to say she looked sorry or he admitted he was wrong. Repentant sits opposite unrepentant: the person who will not say they were wrong at all. It describes a state of heart, not a single quick apology.