Collect totem stickers
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Totem stickers are scattered throughout Alola, even in places where you might not expect. If you manage to collect enough, Professor Oak will give you your very own totem Pokemon that you can use in battle.
All the totem ‘mons you fought throughout the game will be available to unlock, making sure the sticker hunt is worth your time. Looking for the totem stickers is also a great way to explore Alola as you might end up discovering some things you never noticed during your initial Pokemon Ultra Sun and Ultra Moon playthrough.