i need help...due in three hours
Okay, this is probably beyond your deadline, but it is a good question.
Cal lives among the whites during the day and among the blacks at night. She can pass news from one society to the other.
Cal also talks both languages. She can speak properly as white (i.e. educated) people do, but she also uses "black" language around the Negroes so that she won't appear uppity.
Her role in the Finch family is surrogate mother to the children. She not only takes care of household tasks, she is raising the children. She does so "according to her lights," as Atticus puts it, and he trusts her to do a good job.