Though Christabel's late mother is only alluded to in the poem, one still feels her brooding presence throughout. If Geraldine is the representative of evil in "Christabel," then Christabel's mother represents the good, providing a stark contrast to the forces of darkness personified by this "damsel bright."
Sir Leoline's standing order that the castle bell should be rung each day in honor of his late wife ensures that the memory of Christabel's mother lives on, watching over her family like a guardian angel, protecting them from harm. But he lacks the same kind of spiritual connection to his wife that Christabel seems to have developed.
It's rather telling in this regard that when Christabel begs her father to send the evil Geraldine packing, she does so in the name of her mother:
By my mother's soul do I entreat
That thou this woman send away!
By invoking her mother's name in this way, Christabel is calling upon the supernatural forces of good to drive away the supernatural forces of evil as represented by Geraldine. That her father doesn't accede to her request—in fact, gets rather cross about it—indicates once more that Sir Leoline's veneration of his late wife's memory is ritualistic rather than spiritual.