This is obtained upon acquiring 855 different items in the game. This isnt too hard to pull off but it is time consuming. There are 949 items total in the game. Basically any item you can get in the game counts towards this whether it be Gels, Weapons, Armour, Accessories, Rings, Boots, Consumables, Treasures, Quest Items, Materials, Potentites, Valuables or Fashion.
There are:
223 Fashion items
183 Materials (31 Consumables, 22 Enhance Materials, 60 Foods, 70 Salvaged Materials,)
126 Weapons (21 Blades, 21 Papers, 21 Daggers, 21 Guardians, 21 Spears, 21 Bracelets)
126 Accessories (21 Belts, 21 Talismans, 21 Bags, 21 Pendants, 21 Earrings, 21 Ribbons)
81 Potentites
63 Valuables
42 Armour (21 Men's Armour, 21 Women's Armour)
31 Footwear (11 Boots, 10 Men's Shoes, 10 Women's Shoes)
30 Recipes
24 Rings
20 Expedition Treasures
Fashion Items: These are all awarded for Completing the Main Quest and Side Quests, Opening Katz Boxes and winning Mini Games\Item Exchange. Most the items can be bought with Tales Coins so I would not focus too much on the Mini Games. Skip the Mini Games except Slash Beat, Chamballoon 2, Card Game, Waiter\Waitress Game and Fishing(Save before fishing as it is a random number game. Each location has a special fish, use the rare bait and reload until you get it).
Weapons, Accessories, Armour, Rings and Boots: These are self explanatory. Most can be bought at Shops by the end game and others will be in chests. Some Items can only be obtained in the Bonus Dungeon. If you are missing items check the Enemy log and see what Item drops you are missing and fight those enemies until you get those drops.
Recipes: They are all obtained via the main quest and the side quests. Be sure to remember to activate Kamoana's side quest in Meirchio after completing Eleanor's side quest.
Potentites: These are mostly dropped by Bosses and Code Red Daemons. There are a few given as Shop Rank Rewards, completing Class 4 Islands and You will acquire one for dismantling a +10 item.
Valuables: These are mostly story related. You will also need 38 Katz Boxes Opened, complete the Coin Collector and Flag Collector Mini Games, Fishing Mini Games and Find "Terror Island"
Expedition Treasures: Simple enough, there are 20 Islands and each Island has a treasure. Scout each Island until you find its treasure
Materials: These are all obtained in Chests, Glowing spots, Shop and Expeditions