Dorcas is a character in the Book of the New Sun series by Gene Wolfe. She is probably named for the Biblical Dorcas from Acts 9:32-43 for reasons that should be considered spoilers.
Significance
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Dorcas is inadvertantly resurrected by Severian at the Lake of Birds. There are hints that she has been dead for some time, though she appears young. (The Biblical Dorcas was also resurrected.)
She is almost certainly Severian's paternal grandmother. Charon (the old man at the Lake of Birds) and Dorcas had Ouen, the waiter at the Inn of Lost Loves, who refers to Dorcas as his "mother come again." Ouen later tells Severian that a locket he has contains a picture of said mother; Severian recognizes it as Dorcas. The innkeeper there then notes that while Ouen does resemble his mother's picture, in profile he's very like Severian.
Severian quizzes Ouen about his past loves, asking "A woman you loved--or perhaps only one who loved you--a dark woman--was taken once?" Ouen confirms that a woman named Catherine was taken by the law (and therefore handed to the Torturers) after having run off form some religious order (probably the Pelerines). Catherine's child was raised by the Guild.