To take a break from all the angst im dishing out, i like to believe Na'vi and Humans have more fundamental phisiologic difderences beyond just "they are bigger, have tails and also can connect with animals" and by that i mean plot devices to make Spider stand out positively + fluff.
For example, pandora has a very stable, very warm climate, most of the planet being jungle/plains, so it would make sense for the Na'vi to not be all that adapted to extreme heat or, much less, cold. And, in awa'atlu, we see that things are considerably colder because of the sea and lack of protection against wind, this is backed up by the fact it seems every marui has a cookfire/heartfire unlike the Omatikaya that slept on hammocs without any protection at all against the night chill (because they didn't have to deal with cold since the forest was always warm)
Na'vi bodies could very well be almost cold blooded exactly because there was no need to thermoregulate a much if the enviroment is always warm but not overly so.
The forest Na'vi simply didn't have to deal with the cold and the sea Na'vi use their fires to warm themselves up before swiming, kinda like marine iguanas.XxxxBut you know who can regulate his body temperature pretty well? Spider. So imagine the blessing that it is to have your, conveniently sized, family member to be a warm and soft and squishy little teddy bear that also JUST so happens to have become a bit touch starved after your parents may or may not have left him behind?
The sully kids would fucking fight to cuddle with him, because not only is Spider a fucking blessing against the cold nights of awa'atlu, he is also soft and quite willing to snuggle back, and, unlike all of them, he is the last thing that still carries the smell of the forest. Tainted by chemicals and saltwater? Yes, faint? Also yes, but if you just close your eyes and breathe in you can smell the moss and flowers and ferns of home.
Spider is like, the perfect pillow/snuggle partner ever because he checks all boxes. 1 warm (human body metabolism go brr), 2 soft (unlike the Na'vi that have very little body fat Spider is soft and squishy compared to them) 3 will cuddle back 100% of the time (after so much time in cold RDA cells he clings to whoever is close to him and doesn't let go) 4 smells like home still and 5 that is basicaly exclusive to them, Spider knows the sully kids, and therefore knows how to make them fall asleep faster (remember, he IS the oldest) he knows how to massage their scalp just right to make them melt, what songs and humming put them at peace, how to comfort them if they had any nightmares, anything, Spider just knows them too well.
Soon the Metkayina hear about the "coldless" child, and Aonung does bullshit his way into convincing his parents to let the sully kids (+their little heating pad) sleep in their marui.
Ronal finds a pile of blue and teal on the ground in the morning with a single small pink hand poking out and a very strained "help me" coming from under the snoring teens.
The only two people that are somewhat unhappy with it are Jake (bc now he is just the second warmest and his kids more often than not go to Spider in the night instead of him) and Neytiri (for obvious reasons, but it would be funny if Neteyam survived and was specially vulnerable to the cold so it became Spider's unnoficial job to cuddle him through the cold night and she just has to endure it bc if it was jake or herself doing so it could hurt him or wouldn't be warm enough)
