The Good Thief was one of the two criminals who was crucified with Jesus. One of the thieves joined the Roman soldiers in mocking Christ and demanding to be saved, while this one said, "Remember me when You come into Your Kingdom." Jesus assured him that he would be with Him in Paradise that very day.
He is known in the West as St. Dismas, which means "sunset" or "death"; while the Eastern Orthodox tradition simply calls him "the Good Thief" or "the Penitent Thief." He is given other names in a few other places. He is usually depicted on a white background and surrounded by trees, which represents Paradise or Eden. He is commemorated on Good Friday in the East; but on March 25 in the West, based on a tradition that Christ (along with the Thief) died on the same day as the Annunciation, which is March 25.
His couplet in the Greek Synaxarion reads:
Eden's locked gates the Thief has opened wide,
By putting in the key, "Remember me."
His words are also quoted at every Divine Liturgy, at the end of the pre-communion prayer: "like the Thief will I confess You: Remember me, O Lord, in Your Kingdom."