I don't mind a forgettable cast in an horror story. Your ''allies'' are usually meat to be grinded, and that's it. And, to give it credit, the cast you meet in DS3 is quite good. You just don't get to know them. Except for that faux-Jeff Goldblum/villain.
To be honest, after beating the game once, I felt depressed about it. It's when I booted it again in Classic mode that I found what I loved about the series to be, mostly at least, back. No more gun-making, no more abundance of everything. Just you, your normal guns which you need to craft from scrap, and enemies that hurt. I'm just about to reach the ice planet, and I just got my very first new gun. It made the whole thing relevant now.
As for Carver, well, at least he's got a face, voice and backstory and his co-op missions are quite good.