Use the adjective merciful to describe someone who has compassion for other people, especially when he is in a position to punish them or treat them harshly.
If you're caught cheating on a math test, your best hope is that your teacher will be merciful, or that she will forgive you for what you've done. Forgiving someone or relieving a person's pain are both merciful acts. The word merciful also has religious connotations which come from the root word mercy, used since the 12th century to mean "God's forgiveness of his creatures' offenses." The origin is the Old French merci, "pity or thanks."