Who is Agatha in Saint Maybe?

Agatha is the eldest child of Lucy Dean Bedloe, and she is described as serious and unattractive. She takes on a motherly role to her own mother and her siblings, and she often takes on responsibilities that are beyond her years. This is a result of her mother's fears and inability to handle the stress of single parenthood. Lucy takes sleeping pills to help deal with her hard life as a mother and her inability to find another man to take care of her and her children. Agatha sees and understands that Lucy is ill-equipped to act effectively in a parental role, and so takes on the role herself as best she can. This fits excellently into one of the recurring themes of this novel, that the "perfect family" is a myth. Hope this helps.