There is no story reason for it. Halsin was never intended to be a companion, hence "the wall" in act 1 and 2.
But some people thirsted constantly for Halsin sex during EA that Larian decided that they could score easy points, especially as sex became the main selling point of the game when everything else failed.
So the story was rewritten and Halsin became a companion in act 3 (he was supposed to stay in act 2) and as only character trait they gave him "polygamy bear". But Larian did not rewrite the old and already finished content where Halsin is not sex crazed.
Pretty much. Personally (especially as a small halfing who had no interest in Halson) I found his sudden shift from being pollite professional, to sex addict rather unsettling.
That and he is actually pretty boring as a companion. He doesn't bring much to the table. A lot of NPCs you meet along the line, would make more sense as companions, because they actually would have plot hooks in act 3: Barcus of course and the whole plot around Gortash and the artificer guilds, Korrilla and her ties to Raphael ( and maybe her sister as a bonus companion depending on your choices, similar to Minsc), Kagha could have had whole redemption plotline for her, a lot of the cultists in act 2 could have made sense, since they probably would be eager to see the Absolute gone too, and maybe pick some bones with Gortash and Orin. Hells, even Abdirak would have had more stuff to do for him with the Ilmater plot and the Chosen of Bane thing as a Loviatan.
I agree that Halsin was pretty much chosen as eye candy and not much more. Larian pretty much sold him as the resident sex toy so to speak. While every other companion has a lot going on for them, even outside romances, Halsin only has the romance in act 3 going for him, nothing else.
Meaning I agree with Ixal and Wormerine, that Halsins inclusion into the companion cast wasn't a planned thing, but a marketing coup , since people were lusting for him. I think act 1 and 2 were Halsin as intended and act 3 was for the simps, therefore the story doesn't make a whole lot of sense. He is even still listed as an ally in the final fight, but you can't call on him or the grove ( as was probably the original intention).
