Opal Grotto is the caves in Emerald Forest in the northern cliff, there a few entrances.
You need to give her [Gifts], things like flowers and gems. Go to your inventory, pick any item that has [Gift] written in the description and select it. Your character will now be holding a present, then talk to the NPC and an option will pop up to give the present to her. You need to build up her Affection to ~30 before she gives you more quests.