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If you fight echoes in an area, you should see a rare encounter combo counter start to increase in the top left corner, with an image of a girl in it. Keep raising the combo until gold echoes appear, those are the ultra rare encounters. When you defeat one, you unlock the corresponding figurine. Sage is a specific character cosplaying as a Serperior, she appears in the forest just past the door that is locked at the beginning of the game (unlocked after beating the ranger girl). If you go back to the dumpster and go to the south east, there should be a small pyramid with echoes around it. The ultra rare encounter here unlocks the old Mara figurine.