Partly, of course, this is a definitional problem.
If you say that hard SF is the stuff with the technology and science playing the definitive role, and the characters are there to put it into action, you will end up with cardboard characters. Or if you expect the attitude of "The Cold Equations." But if you stick faithfully to science and even solve an equation to write your novel, you can call it hard SF and still have characters.
There's a lot to do with the impact of the science and technology on the characters.
There are limits in who you can have in a hard SF story, because you might end up with the Pern problem and people not realizing that it's hard SF or even SF at all. This is why you have so many scientist main characters.