I've been thinking about this. You know how part of the problem with the sandbox discussion was that different people used "sandbox" to mean different things? Well I think we're using "personality" in a couple of different ways, too.
Let's look at the Mass Effect examples. Shepard gets three responses to each option - roughly corresponding to good/neutral/evil in D&D terms. So some folks are taking the enirely sensible view that the Shepard who's willing to do something horrible and selfish in a given circumstance has a different personality to the one who's willing to say nice things in order to get to exactly the same place. It's a reasonable way of looking at it. I mean Shepard doesn't know his choices are meaningless, after all.
On the other hand, I don't think it's the most
useful way of looking at it. For the purposes of this disucssion, it's better to think of Shepard as a man (or woman) who like most of us is capable of both good and evil, and makes moral choices on a case by case basis.
Looked at that way, Shepard has a fairly clear, singular personality. He's got your basic miliary mindset. He's goal oriented; a team player; no discernable sense of humour; he thinks IP licences are a really good idea (I wonder why) and he's a wee bit clueless when it comes to getting laid. That's how I think of Shepard's personality. The Good/Neutral/Evil options are better thought of as morality, and from a game design viewpoint, they are complimentary to the matter of personality.
In morality terms, I'd like to write events that (like Mass Effect) offer a range of moral responses. Unlike ME, I'd like them to make some sort of difference.
In terms of personality, I see the MC as having a clearly defined persona. He's a tough guy, but not brutal or mindlessly violent; he has a sense of humour, tending towards the dry, larconic end of the spectrum; he has a strong sense of right and wrong, and like many real world criminals, he's capable of getting upset by any moral outrage that he hasn't actually caused.
In a lot of ways, he's like a Raymond Chandler PI, dropped into a fantasy setting and working as a criminal rather than a detective. There's a quote from Chandler that puts it nicely.
But down these mean streets a man must go who is not himself mean, who is neither tarnished nor afraid. He must be, to use a rather weathered phrase, a man of honor -- by instinct, by inevitability, without thought of it, and certainly without saying it. He must be the best man in his world and a good enough man for any world... He will take no man's money dishonestly and no man's insolence without a due and dispassionate revenge ... and his pride is that you will treat him as a proud man or be very sorry you ever saw him. He talks as the man of his age talks -- that is, with a rude wit, a lively sense of the grotesque, a disgust for sham, and a contempt for pettiness.