There's a fishing reward at a certain quest I complete. What if I get a lot of those fishing quest items and created a lot of accounts, I then gave them one by one with each character. Do I still get those rewards like the golden fishing rod after a a certain amount of fishing quest item I gave the angler(NPC) ?
The Angler rewards based on number of quests completed has nothing to do with the items a player has. It's actually part of the character's data. So when you have something like the Golden Fishing Rod that is given out after the 30th completed quest, it is given out after that character completes 30 quests. Regardless of items they possess or the world they are completed in.