I took the letter from the bone cave and received it often from the doctor. I built and upgraded the castle. I visited the people in the camp. What should I do next? I've been stuck in this part for 5 days already
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Just as I mentioned in the previous post and if the screenshot you posted is correct, you need to get a fire crystal for Ayane the kitsune. She's the fox girl in the temple who will help Ada in her journey.
The crystal in the desert zone, the one before the puzzle map with a lot of roots/vines. Climb them down and open the chest. There's only one such map in the desert so you can't really miss it.