Being diminished into a low level companion ruins Halsin as a character for me, unfortunately. I won't be able to take him in my party.
Nettie would have made much more sense as a companion as Halsin should be at least level 9 to reflect his status as First Druid and "powerful healer". The world needs powerful NPCs to be complete. Larian are telling us no NPC can be more powerful than your party, ever.
The complete disconnect between game mechanics (levels) and storytelling in BG3 is appalling and unnecessary.
They could draw inspiration from their own titles, I don't know if you've played DOS1, but there were several NPCs there who were god-tier yet appeared in the game early. I mean ZixZax and Arhu, they were both level 20+ and you needed to grind your way through the whole story to take them out. They could demolish you in a single turn therefore cementing their status as 'powerful characters' any day, unless you approached with like 100% resistances or 100 initiative. Unfortunately when it comes to companions, Larian sometimes fell short, just look at Jahan from the same DOS1, who had an 'almighty-immortal-king-demon hunter' backstory yet was level 1... Same story could happen with Halsin, however they could pleasantly surprise us as well.