On one hand, this is an easy question. Pick a character and defend your opinion. On the other hand, this question is tough because none of the other characters are meant to create feelings of sympathy within a reader.

As a parent myself, I would immediately lean toward sympathizing most with Gregor's parents. Their child has been turned into a bug. The son that they have known his entire life is completely different and unable to communicate with them as he used to. They are repeatedly disgusted with the sight of him and embarrassed enough to try and lock him away in a single room.

I also feel a fair amount of sympathy for Gregor's sister, Grete. I feel sympathy for her because she takes caring for Gregor completely upon herself. Her parents are next to useless when it comes to caring for Gregor. They are more or less completely unwilling to take up the task. I'm sure Grete is not overly thrilled about having to take care of her bug brother, yet she does it. She is not get rewarded with anything for her efforts.

Finally, you could probably make a sympathy case regarding the cleaning woman. Some of the same reasons to support Grete can also be applied to the cleaning woman. She takes on the job that nobody else wants. Unfortunately, as a hired worker, she is easily dismissed by the rest of the family.