If I had to pick a short character currently in the game to be a companion, I think it would be Thulla. If not Thulla, then possibly Nettie.
As much as I like Barcus, I agree that it would be harder to transition him over to a full companion.
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Scratch all of that. If I had to pick one, I'd pick one of the goblins. I almost don't care which one because I love them all, but Sazza probably makes the most sense.
But if I couldn't have a goblin, then I'd go for Thulla. After her, Nettie.
By the Gods, anything but Nettie. I understand her point of view during our first meeting but her assassination attempt and incitement to suicide shortened our relationship. :p