• Accessories works only for the character equipped.
  
• You can equip two of the same accessory, same or different level will add up.
  
• The character need to be out for the accessories to have their effect.
  
• One exception is the Growth Pendant. It actually affect only the character with it equipped but the character does not even need to be out since everyone get experience. It has nothing to do about who kill the enemy, last hit or anything like that. Works only on combat experience, not quest rewards.
  
• Not accessory related but, while we're into experience, a 0hp character will not get combat experience. Otherwise as long as they have HP they will get experience even if not active in combat.
  
• Putting the trap with the character equipped with the Hunter's Earring seems to give more and better meat overall. It would also follow the same rule as the other accessories so that checks out.
  
• The number of button prompts while fishing make it easy to confirm the Fisher's Earring works only on the character equipped.
  
• The Pioneer's Pendant, that gives more enemies drop, actually gives Luck. Combined with my tests I think the character with the highest luck is overall better for monster basic drops. Giving all the Luck food to the same character is probably recommended if so.
  
• Luck affect the monster drops, the inn Luck bath description confirm it. I'm not sure about the rest of the resources and how much, if at all, it affect rarity of drops.
  
• The character out by the time the drop appears seems to affect it. Not last hit or kill. I used the Rarity Pendant on a second character to test this and even when doing nothing getting this character out gave me more rare drops. Juggling the enemy at 0hp, hitting it with a Link Attack or anything else did not seem to matter.

I spent about an hour doing runs of the some of the Forest areas with different equipment, lvl 3 so a 15% increase should be somewhat noticeable, to see what change drop wise for the accessories.

• Covetous Pendant, Rarity Pendant, Pioneer's Pedant and Prospector's Earring on a character not in play did nothing as far as I could tell.
  
• Covetous Pendant: Maybe 1 in 20 enemies gave me double of their basic drop, never happened for their more rare drop, even with a Rarity Pendant also equipped. Works on some destructible like mushrooms and crystals with the same low rate. Does not seems that useful since you'll get tons of the basic stuff.
  
• Rarity Pendant: Seems to work on everything that has rare drops, enemies, pots, boxes. What I'm not sure are Rocks and Trees but I think it's all about the tool level.
  
• Pioneer's Pendant: By itself seems to give more chance for the basic loot, very rarely got anything rare without the Rarity Pendant also equipped. Both equipped seems to help gives more loot, probably need to have a basic loot in the first place for rarity to upgrade.
  
• Prospector's Earring: I equipped two of them and it was very negligible. BUT when doing the test I had lvl 3 tools and already get a lot of rare drops from the rocks.