I've been too busy for proper Outer Wilds thoughts so far today, so have a list of random character headcanons instead:
Slate used to be bad at self care while working, but when they noticed Mica was mimicing their habits, they got too icked out by the idea of actually being the bad example Gossan thinks they are and improved.
Marl claims they can "smell the rain in the trees" before it comes, and the younger hatchlings are enraptured by the wisdom. Rutile definitely saw them sneaking in to check the orbital satellite for rainclouds one time, but as long as the gaggle of awestruck hatchlings keeps the treekeeper from acting on their grudge a little longer, they're not telling.
Hornfels secretly writes poetry. The rhythm and the rules of it appeal to their scientific mind, and it's a good outlet for their curious musings and frequent bouts of sadness - but they're a little embarrassed of it next to Gabbro's quantum display, so they keep it to themself.
If a signalscope goes missing, anybody looking for it immediately goes to Tephra. They're indignant and often innocent...and it handily distracts whichever stressed-out traveler it is this time from the fact that Galena is the one who's really stealing them.