Secret or Hidden Areas that require Kai's Mask
There are a number of areas that remain inaccessible until Kai gives you his Mask. One of these is the Autumn Fields entrance to Clarice's secret base.
Another is in the Winter Wilds. Go to the Forest Depths Save Point. There is a rather narrow path between two high mountain areas. Follow it east
until attacked by a Grizzly Bear. Look to the left for what I describe as a Kai's Rock that only can be smashed using his Mask. You will find an Underground passage filled with mushrooms.
I found it in the course of looking for Pilika's Event where the little girl is saved from the Blizzard Wolf by the Bear.
The Blizzard Wolf is Lv 50 and you will engage with it briefly.
Likewise there are many areas that require the Parasol. The first and most obvious one is to reach the Winter Wolf but it is required forva number of places in the Winter Wilds.
Sometimes wll it takes is daylight instead of night. In another game, I had reached the Winter Torii Gate effortlessly. It was only when I decided to ise the Parasol at the Crevasse again that upon landing and running to the edge of the cliff with the Frog you see immediately that I saw the Torii Gate physically and a wind current that led to it. The Save point at the end has a chest beside it that contains a Rune Crystal. They are rare!
The chest beneath the arch there simply gives you a Snow path square.