Since you know like 10000000x more about the elven language than me, what are your thoughts about the meaning of the name Halsin?
I like this question, because based in interpretation*, the name does suit him! We're going to use what we have access to in DnD elvish, as Tolkien elvish is a different language entirely and they don't have much overlap.
We'll have to break his name into pieces, and that can be done a variety of different ways, depends what you choose to do. I figured easiest ways are:
Hal - sin
With the "l" being an adjoining tonal letter. In both cases, "sin" would likely be broken up into a combination of "sy"or "syy" + "-in"
Now the meaning of the pieces:
Hal/Hol: "pale, weak" OR "precious" (Side note: Astarion's "The Pale Elf" might be said "Hal'kesir/quessir" - very close to "Tel'quessir")
Suffix -in: "brother, sister, sibling"
So, directly, it could mean:
Hal-sin = "Pale/weak or precious wild brother"
Ha-l-syin = "Free wild brother" (Which I would phrase more like "Brother of the free wild" or just "Brother of the wilds")
I am inclined to lean towards the latter definition/meaning, as I believe it suits him very well, and aligns with the relationship between him and Thaniel. Naming himself as a sibling of the free wild indicates that his relationship with nature is the forefront of his personality (nature being his very first friend), and indicates a closer connection than others might have. Judging by how Halsin speaks of his connection with Thaniel shaping who he was as a person, this may imply he's named himself after how intertwined it all is to his being. Familial familiar and inseparable.)
(It also sounds more in line of what he would name himself than the alt. - I wouldn't really assign the words "pale" or "weak" as something Halsin would name himself...
...unless he is implying, as he was most likely at the point where he was the last of his kin and the only child left, that he was the "weaker" choice of sibling to be left of their line. So "weak brother of the wild". Pales in comparison. But I feel like that's a stretch.)
FUN BONUS, in elvish, Halsin could have written his name like this:
Or like this (A variant of Espruar, the alphabet of the moon elves that was used to transcribe elvish, used around 1372 DR - since retconned. And since Halsin would have likely learned to write between 1147 DR and 1152 DR, probably not used by him, but possible he learned as he would have been around 120 by then):
*Disclaimer that DnD elvish, at its core, has had a very contradictory lexicon over the many years, and most words are drawn from 2e and 3e materials - elven also has several words for singular things, single letters that can change the meaning of a word or sentence entirely/thrown in to make a word flow better and have no meaning otherwise, and is a very tonal based language in that you could physically be saying one thing, but another elf would hear something entirely different in the intonation. Therefore, everything I write here is interpretive based on the materials we have access to, with a little extra HCing and logical assumption.