Protagonist
Character Role Analysis
Susan Calvin, Stephen Byerley, Gloria Weston, Gregory Powell, and Michael Donovan
Here's the thing: since we have different characters in several of these nine stories, we're going to have several different protagonists. But since none of them stick around for a long time, it's probably worth considering them all together.
Now, it's not always clear who the protagonist might be in these stories. There's a rule of thumb that says "look for the character with the most to lose"; but there's another rule of thumb that says "look for the character you care the most about." But if we really looked at these stories, we'd probably find a new rule of thumb: the protagonist is the smartest person in the room. That means that Susan Calvin is always going to be a protagonist—because she's (almost) never wrong and always comes to the right answer quicker than anyone else. Similarly, we might consider Byerley as the protagonist of "Evidence": not only does he have the most to lose (an election), but he's smart enough to win by turning the villain's dirty trick against him.
So, in an Asimov story, it's important to be smart.