Who is Louisa Parker from Sharpe's Rifles: Richard Sharpe and the French Invasion of Galicia, January 1809 and what is their importance?

Louisa is nineteen-years-old and longing for a more adventurous life. She is engaged to be married, her fiance is wealthy, and her aunt is taking good care of her in the meantime. Louisa, however, an impulsive, young woman, makes decisions that quickly change the course of her life.