Laura Lyons is the daughter of the Baskervilles' neighbour, the crochety Frankland, and is also the woman who unwittingly lured Sir Charles Baskerville out to his death. When Watson goes to visit her he describes her as follows:
The first impression left by Mrs. Lyons was one of extreme beauty. Her eyes and hair were of the same rich hazel colour, and her cheeks, though considerably freckled, were flushed with the exquisite bloom of the brunette, the dainty pink which lurks at the heart of the sulphur rose. Admiration was, I repeat, the first impression. But the second was criticism. There was something subtly wrong with the face, some coarseness of expression, some hardness, perhaps, of eye, some looseness of lip which marred its perfect beauty. (chapter 11)
Laura, then, is beautiful, but also there is something 'subtly wrong' in her appearance, something that hints at a rather coarse nature. Her manner is rather abrupt and unfriendly.
However, Laura has had a hard struggle. Deserted by a faithless lover and disowned by her father, she was left in dire financial straits until Sir Charles offered to help her out. Meanwhile the villain of the story, Stapleton, who wanted to get rid of Sir Charles, seized on the opportunity to use her as bait to draw the old man outside. He persuaded her to write Sir Charles a letter asking to meet him in the grounds of Baskerville Hall, presumably in this business connection. He then prevented her from actually going, so that Sir Charles was left alone outside at night, where Stapleton let loose his fearsome hound on him, literally frightening him to death.
Therefore Laura, although wholly innocent of any intent to murder, plays a crucial role in Sir Charles's death. She is manipulated by Stapleton, who appears cunning and unscrupulous to a quite extraordinary degree.