I think the 'Obligatory Poly' in the title of the thread pretty much sums up what's happening with Halsin.

The writers were tasked with making SOME kind of polyamory present in the game's relationships, and unfortunately, they sort of jammed it into Halsin's romance options awkwardly.

I simply don't buy this particular guy as being a poly. I really don't buy him being a manipulative prick either. It seems that originally the writers wanted this to be a sweet bara daddy type of guy, but then looking at their cast, they realized there wasn't enough poly representation, and people are OBSESSED with representation these days. Halsin was already sufficiently catering to the furry crowd, and they could have done well to leave it there.

Even having him ultimately not a full-romance option would have been fine, he has had some very traumatizing experiences in his backstory, and being mistrustful of his own feelings, and of people romancing him, is totally believable. He could even have offered sex, but not a more complete relationship, because he loves nature enough to be in tune with physical passions. "Let's take it slow on the feelings, but if you want a little fun for now, I'm game" is a totally valid position, and would even have helped him seem mature, which is how he is portrayed in the earlier segments of the game.

Instead, he, like most companions, seems all-too-willing to fall in love with your Tav. That's also fine, but trying to make the DRUID the poly one makes the LEAST sense.

The way he acts currently reminds me in a lot of ways of Minthara, who also has some really weird dichotomies in her relationship with Tav. It makes SENSE for her to be manipulative and dishonest about whether she wants feelings or just to get sticky with you.

I think the real fault here is that the writers felt FORCED to include the 'poly' aspects of Halsin's romance in order to cater to a very specific crowd, and he's just not the right character for it. As a non-origins character, he doesn't have as much time in the focus of the story to fall back on to flesh out his romantic story. As far as I am aware, all of the non-origins characters are poly-friendly, except where they conflict with an origins-character who explicitly dislikes them.