As others have mentioned - Characterising Halsin, as currently written, as Poly is harmful and a destructive misnomer that does a fairly bad disservice to people who do live in poly situations. He's relationship averse, and if he refuses to think of or call what you have a relationship, then it's certainly not a poly relationship.

As you say, what he seems to be, by what everyone describes, is a strictly non-monogamous and likely aromantic person... though that casts some shade on some of his other behaviour and comments which kinda gaslight the player as having been romantic with him, over simple friendly or just plain 'what any decent person would do' actions.