This is just a general worldbuilding question about your GMAPS series.
Nyll didn't want to show his true form around the other students, but given how some of them seem to have mythological heritage why would he want to seem more human? Wouldn't that almost paradoxically alienate him more?
Sorry if the reasons been stated somewhere already 😅
Nah it's a good question! The simple answer is that hobs (which Nyll is) do not exist on this side of the world, outside of bones that very rarely wash ashore from the ocean.
For the lore dump answer...
A long time ago (before Lobolg's birth, even) the gods of the time divided the world with a large ring of storms, what most people call The Tempest. Nobody is certain why it is there, but it's essentially blocked off travel from either side of a quite large ocean for hundreds of millennia. Attempts to sail or fly through the Tempest are a death wish, and teleportation is blocked off as well. Given the difficulty of ocean travel even before the Tempest rose, the native people of each side soon became isolated to that side, stories about them on the opposite side of the world mere myths and hearsay.
Here's a map of the world if you wanna see!
Merlen is on the right, while Gala is on the left.
The story of GMAPS as a whole takes place in Merlen. It's fairly similar technology-wise to our own world, save for space travel (which is blocked by the Tempest, as usual), and it's been in an era of peace for centuries now, with the only wars breaking out being resolved within just a couple years. In particular, our focal family is living in the province of Lydia, in the city of Merlopolis. It borders a region predominantly populated by giants, hence the larger-than-usual population of them for a small region bordering the Merlevinian Sea.
Earlier in the chapter, though, Nyll and Asteria called Gala home, the other half. Gala's been facing incredible strife for a long time, and Nyll's people, and Lobolg's progeny, the hobs, have faced the most discrimination of all. They've been mostly pushed out into the Lohobvari peninsula, which sits close to the Tempest. I've circled the area they lived in below.
The only people groups each side have in common are humans and merrow, given the former's status as the first people, and the latter's ability to travel the seas long before the Tempest barred them from moving. Now that hundreds of thousands of years have passed, though, these people would hardly recognize each other.
So, yeah. Nyll can't look like himself because Merlen views hobs as monsters of myth and rumor that go bump in the night, further amplified by the fact that they only came into being after the Tempest. Their sheer existence as people is speculative, as all that Merlen has seen of them is bones, making ascertaining whether or not they were sapient a difficult task.