Miranda was the maid to the Dark Sisters. She never really had a big part in Tessa Gray's life there, only bringing her food and taking her to see the Dark Sisters. Tessa tried to question her numerous times but she never responded to her.
In an attempt to escape, Tessa hit her over the head, ripping off her flesh and revealing that she was actually clockwork. However, she did not completely destroy her as when Will and Jem arrive at the Dark House to investigate, Miranda sprung to "life" and warned the pair about things to come before seemingly shutting off for good. Henry Branwell later examined her body.
Tessa says Miranda didn't say much to her and never answered her questions. She seemed unaffected by the Dark Sisters and Tessa never really understood. It was later revealed that this was because she was a clockwork creature.