In order to access this quest, you must first fix the Love Engine so that the residents of Valentown are friendly again. This is necessary for the story. After this, you can start accepting side quests from the town folk.
You need to speak to a woman named Ellen. She's located in the north-west part of Valentown (walk through the area that looks like a cave). After you've accepted this quest, there are several steps:
1. Find an Uncharged Crystal for her boyfriend's soul. The crystal you need is located in a blue chest at the Mirrocle Caves. This particular chest is in the "reflection" room, where you must direct beams of light to unlock the exit. The chest will only open if the light is reflected on to it.
2. With the crystal uncharged, you must find a way to charge it. Lady Ayumi (Queen of Butterflies) at Butterfly Hills is who you must visit. You can either use a Recollection Orb to go there (assuming you've unlocked the portal), or you can travel there from the Ladybug Village if you unlocked the teleport there. Alternatively, you can travel south from Valentown, through the Lands of Love back to the world map. Back on the world map, go south to find the Ladybug Village. Heading even further south will bring you to the Butterfly Hills. You must navigate through the cave system to find Lady Ayumi in her palace.
If you've already done her side quest earlier on in the game, it will be easier to navigate, and she will charge the crystal for you upon talking to her. If you have not done her quest earlier, you will have to do it now before she will help you. At this point, however, it is TRIVIAL with the low level monsters so you should be able to finish it quickly. See "The Queen's Gift" achievement for details on her quest.
3. With the charged crystal, you must return to Ellen in Valentown and speak with her. She will tell you that her boyfriend's soul is in the Sweet Tower. The exit leading to the Sweet Tower is to the east. On the world map, travel north. Travel north on this screen to reach the Sweet Tower (a dungeon we already previously cleared).
4. Once inside, go to the Love Dungeon by traveling down the ladder. Once here, go down the stairs and into the first door you see to your east -- a shortcut we unlocked earlier. You will exit into a hallway with a switch to your right. Go north east, following the path to a ladder. Head north east again, and go through the doorway with torches on either side. On this screen, head north at the first fork, and head right at the second fork. You will see the spirit of Ellen's boyfriend being guarded by monsters. Defeat them and pick up the soul.
5. With the soul, backtrack to Valentown and return to Ellen. You will be awarded with the Love Ribbon and the achievement for completing the quest.