Just plain blame @bloopsalot for all of the soulmate things they're taking over my brain
Humans have soulmate poppets. A plush made of cloth and felt that represents your person. Familial, platonic, romantic- hard to say. But they exist. And the Graces are very, very confused when their son is born with a puppet - not a poppet - made of... rock? But that's their son's soulmate, so it can't be helped
Ryland Grace hugs his soulmate puppet from a very young age. All of his baby pictures have him holding onto it and hugging it to his chest. Chewing and teething on the legs. No one seems to realize it's a poppet, they just think it's a favored toy, and then as he grows he learns to keep it tucked out of sight.
Eridians, however, do not have poppets or puppets. They have *hands* that interlock perfectly. Five hands, each one made for someone who you love and will be important to you. Five soulmates one way or the other. Imagine being born knowing that one of your hands is forever made to fit perfectly with your parents'. Imagine parents excitedly comparing their identical hands meant to hold onto their children. Imagine growing up looking at your hand and knowing you will hold your child with that hand one day and it will fit perfectly.
There are also romantic soulmates too, for eridians, and Rocky finds Adrian to hold onto. Two puzzle pieces coming together to hold while sleeping perfectly safely.
Except Rocky has always had one particularly strange hand- and they are delighted to find Adrian does as well. Made of a soft rock that chips easily and doesn't transfer heat very well. It's always *cold* for some reason, worrying almost, but it doesn't hurt either of them. They've been to doctors. Their fingers are more rounded, smooth with no sharp edges. Hands that they don't realize are made to hold something soft and malleable.
It takes Grace a long time to meet his soulmate, and even longer still to end up on Erid where he has two hands to hold perfectly comfortably. Then it's pretty clear why Grace always sweats no matter what and why Rocky and Adrians' hands are always freezing (and then over time, Grace wakes up and there's another Adrian colored puppet by his pillow, and he's just resolved to accidentally giving himself head trauma in the mornings on various rock objects and/or people)
