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
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 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
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.