This article contains information on how to unlock the different endings in Dragon Quest VIII. The 3DS version of the game however lets the player unlock a new ending to marry Jessica not found on the original Playstation 2 version of the game.
Marrying either Medea or Jessica
To unlock the two alternate endings, you first need to clear the main storyline and have viewed the normal ending.
Original Alternate Ending: Marry Medea
After watching the normal ending (where the Hero runs away with Medea after crashing her wedding), you need to clear the Heavenly Dais bonus area and defeat the Lord of the Dragovians. If you succeed, you will obtain the Argon Ring. Defeat Rhapthorne again and the normal ending will be replaced with an alternate one where the Hero marries Princess Medea in the cathedral.
New Alternate Ending (3DS only): Marry Jessica
You will also need to clear the Heavenly Dais bonus area and obtain the Argon Ring there. Afterwards, head to Jessica’s house in Alexandria and talk to her mother. You will receive Alistair’s Armor. Then, spend the night at Alexandria’s inn and watch the cutscene. Defeat Rhapthorne again. Then, go and see King Clavius at the cathedral at night. When he asks if you like Princess Medea, answer “No”, then “Yes” and “Yes” to the last two questions. This will unlock the new alternate ending where the Hero marries Jessica in the cathedral.