The Rogue Knight's Sword is probably the best weapon you can get in My Time At Portia, and you will definitely want to use this weapon for your end-game adventures. This sword has +480 Attack, and + 100% Critical Chance, so you won't find anything better than this in the game.
You can only get the Rogue Knight's Sword after you have completed the main story line, and have finished the "Mission: The Final Battle". We won't spoil here, but once this mission is done, you have completed the main quest, even tough there is still a lot of stuff to do in the game.
Anyway, after you beat the boss in the "Mission: The Final Battle", look for the sword, which is sticking up from the ground close to where the boss died. If you forget to pick it up, head back to the Central Plaza and do it later.