Maggie the Thinkamancer was part of a group of three linked casters, along with a Foolamancer (Jack Snipe) and a Lookamancer (Misty). This combination was used by Stanley as an command-and-control centre: The Lookamancer determined the identities and positions of units on the battlefield; the Foolamancer presented this information in a readily understood display; and the Thinkamancer relayed reports and orders.
Maggie also provided the necessary magic for the set-up; only a Thinkamancer can establish a link between casters.
She is described as a "cold fish" by Parson. She is quite effective and provided Parson with a lot of useful information about thinkamacy.
After Lord Stanley ordered the breaking of the link, only Maggie left the set-up without damage. It is heavily implied that she shielded herself and channeled the damage to the other two casters. Therefore Misty died and Jack suffered since then from mental problems.
She was also part of the the short-lived tri-mancer link involved in the Volcano Uncroaking.
Real World References
Maggie is most likely a reference to Margaret Thatcher, to whom she bears some resemblance.