If little is known about the handmaiden Sabé, there is next to nothing known about Sabé the girl, so most of this is pure speculation. Sabé never speaks a line as "herself" during TPM. The few lines she has, she is playing the Queen, so we really don't get to see any of her personality.
What we do know: Sabé is Amidala's best friend. I think that says something about her good character. Amidala doesn't seem like the type who would choose her friends hastily, nor would she choose someone who was not.. well, I don't want to say "worthy".. more like she would not become friends with someone who was not a good person.
We also know that Sabé is fiercely loyal to Amidala because she willingly risks her own life to protect Amidala's. Not just because Amidala is the Queen, but because she is her friend. This shows that Sabé is an incredibly brave and courageous person. She is willing to sacrifice herself for "the greater good", so they say.
In terms of personality, I tend to think of Sabé as the strong silent type. In Queen Amidala's Journal, she describes Sabé as "quiet" and "cool-headed". She seems like the type of person who thinks before they speak/act, instead of running into a situation unprepared. This is reflected in all of the training she under-went with Amidala in order to take her place as the decoy queen. That training also indicates that she is a very patient person.
I would say that Sabé probably lacks self confidence, as she is worried that she will not do a good job playing the Queen. She fears that she will somehow make a mistake and give Amidala's cover away, even though Amidala herself trusts her to do a good job.
What we don't know: everything else. How did she become a handmaiden? Who is her family? How old is she (I've seen her age reported as 14, 16, and 18 in different places)? What happened to her after TPM? The questions go on and on, but I guess the mystery around Sabé's character is what makes her so interesting.