Here is a simple guide describing the location of each guardian statue.
Plum & Concord

Unlocking Plum and Concord is easy. Just advance the story until you unlock their memories.
Blue & Hop
Blue’s grave is to the east of Vista Outpost, in Summer’s End. This is the same area where you’ll unlock the dowsing rod.
Hop’s grave is in Lillyputt. To get to it, you’ll need to use a Repair Box on the bridge. It’s waiting in the town for you to interact with.
Cayenne & Bailan
To unlock Cayenne, you’ll need to use a repair box nearby Concord’s house in the Spring Hamlet.
To unlock Bailan, you’ll need to go to the Treeway. You’ll need to use a weapon (as far as I can tell, any weapon) on a certain section of the foliage around there. It’ll take a little bit of hacking to get in, but eventually you’ll be able to find their grave.



Cane & Allio
Once you make it to Autumn Town, you’ll be given a key to unlock several areas, including the town proper. Cane is right in the center of town.
To unlock Allio, you’ll have to clear the trade route to Spring Hamlet. This involves building a few Repair Boxes. Once you do, Allio’s waiting right there.
Mandy
Mandy’s grave is to the north of Concord’s house, at Wonder Pond.
Thanks to silverskyfeather for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.