Go to the "Shop" or "Collection" menu. If successful, every character should have a glowing gold star indicating purchase/completion. Head straight to versus mode and pick Pain or Minato Namikaze to test.
You can find 100% completed save files from community sources like GameFAQs (search for the Ultimate Ninja 5 section) or dedicated emulator hubs. Naruto Shippuden Ultimate Ninja 5 Save File Pcsx2 All