I think she has very little personality other than that she’s mostly grim and down-to-business. Such an attitude doesn’t usually work without having a more sympathetic justification for that kind of coldness. Sure she lost her parents but so did Luke and Leia and they had some spirit.
Have you thought about the starkly different environments each grew up in? Luke and Leia both had stable and safe family environments and don’t even know what happened to their parents until they are adults. Jyn saw her mother get murdered by the Empire and her father abandon her. Instead she was raised by an extremist rebel who basically became her father figure and taught to become a cold and calculated rebel warrior like him. She grew up in guerrilla war conditions for years on end, likely on the run constantly to evade capture until it basically became the only real life she knew outside of her brief childhood of innocence.
Then he to abandoned her, just like her father did…at least from her certain point of view, until fortunately for the rebellion both he and her father later on had the chance to explain the truth of matters which changed every single aspect of how she had understood her life from when it first changed as a little girl.
So yeah, I’m pretty sympathetic to her situation and can understand her motives and choices at different times in her life as circumstances and her own knowledge of events changes.
.Val