Halsin is my absolute favorite. I love the twist that despite being so devoted to nature, he actually isn't that good at being a Druid, as shown by his belief that undead/vampires deserve to live like anyone else (a distinctly un-Druidic belief; even Jaheira has lines saying she beat the crap out of some vampires once to the point that Cazador warned Astarion to stay away from her part of the city). I love his guilt and trauma, interspersed with his kindness. And I have a soft spot for healer characters. Before I played Baldur's Gate, my favorite video game was Trauma Center, for precisely the reason that you were trying to save people instead of kill them. Halsin is like every character trait I love rolled into one.
I also like when characters have flaws. I don't like when they are all sanded down and have all their edges and imperfections smoothed out.
Some of the things that annoy people about the companions are precisely the reason I like them.
