All of them! Some to a much greater extent though.
I can identify with Sally’s challenges pretty much throughout. I can understand how pressure from her colleagues would prevent her from doing what she knows to be right, even though high moral values are very important to her. Her treatment of Mel and Darren is where my sympathy wanes.
I can empathise with Paul’s struggle to properly move on from his relationship with Katie. If you told everyone you were going to make a new life in America only to come back when it fails, that’s hard to deal with in itself.
Darren’s unwillingness to question Vangelis’ choice seems to me a sign of weakness. I can, however, still identify with his commitment to his results (albeit misguided). His relationship with Sally may not always make her life easier but he is always well intentioned and in many ways he is a very good friend to her.
Amy I have great sympathy for, as does Sally. The final stages of your PhD can be hard and confidence-sapping. I’m not sure she has it in her to be a great scientist, so I think any help that her colleagues give her towards getting an academic job may not be doing her any favours.
Vangelis is perhaps least deserving of sympathy. He brought about his own downfall. He clearly cares for his employees though, and is passionate about science. Both these things, however, have perhaps become mixed up with his ego and desire to be a visible leader.
Even Judith I have some sympathy for. Technicians do vital jobs which can be undervalued, and socially it could be hard to fit in with colleagues who are much younger than her.
I think the characters I identify least with are Sally’s mother and brother, partly for their complete unwillingness/inability to properly engage in anything to do with Sally’s science (or life in her brother’s case).