Crucior

My suggestion is the Ctenophora, it looks like a jellyfish, but it does not belong to the cnidarea family, with fossils dating from the Cambrian period, it would be a shoulder creature, it would cling to the survivor from the back with tentacles, it would be transparent and with bioluminescence. While clinging to the survivor, she would have an ability to camouflage herself and the survivor, making both invisible, she would be found swimming in the liquid element, to be able to tame it would require a tek armor to enter the liquid element safely, the food it would be element ore, for balance issues, I imagine that invisibility could have a 10 second cooldown after becoming visible before being able to use it again, and that when attacking it would become visible again, invisibility time would depend on stamina. It would be a passive taming, we would have to give it elemental ore, it could only mate inside the liquid element, after being tamed, it would not die out of water, but it would be unable to move, just lying on the ground waiting for someone to come to take. In the dark of night or in caves it would glow with its bioluminescence, although it doesn't give enough light to illuminate anything, it would be nice to see it glow, with luminous tentacles wrapped around the survivor's torso and arms. Another ability it could have is when in water, it strives to supply its survivor with oxygen, spending its stamina, only recovering it as soon as it leaves the survivor's back or when the survivor leaves the water, thus, the stamina it is a very necessary status, and therefore it should be used well.