An apocryphal legend tells that he was a partner-in-crime with fellow thief Gestas, and had once encountered JESUS as a tiny tot while attempting to mug MARY and her husband in a dark desert alley.
But DISMAS saw the light and persuaded Gestas to leave them alone. The infant JESUS then spoke forth, predicting his future crucifixion and their own guest appearances in it.
When after many years all this came to pass, DISMAS on his cross cried out in worship while Gestas sulked.
So you can guess who JESUS turned to with the promise: "Today thou shalt be with me in paradise".