There is none, atm. You just go back to the door before the final boss, Zelda style.
Well, not all of them. Aside from her friends who are now evil, she spends awfully little time hanging out with Gramps. He is the only one who was with her before, and kept his memory. He can even relate to the dead friends thing. Plus, she should really try bonding more with Brigid and that other blade girl who shows up. Regardless of their loss of memories, Jin himself says that they are the same as they were. Just hang out and become friends again, dumbass. Its a standard anime plot