Halsin follower requirements
Held prisoner in the Goblin Camp. He will join you if he is freed and you did not attack the Grove. He will become a full companion during Lift the Shadow Curse. Halsin can not be recruited alongside Minthara, barring exploits and bugs. In my playthrough I attacked the grove, saving the Tieflings, but Halsin still helped me kill thegoblins, and joined my camp for the journey to the underdark. Note I had already discovered the tunnel before the meeting with halsin back at the grove, which is what prompted him to ask to join my camp.