The two share a strong rapport. As a Caster, Maggie was probably always aware of Sizemore's "Rock Star" status in the Magic Kingdom. However, they probably weren't all that close until they linked to uncroak the Volcano, and learned just how compatible they really were.
Some level of romantic involvement is implied, but not yet resolved as definitely present or absent.
Sizemore and Maggie have discovered that they can easily link up for casting spells together. This linkage not only boosts Sizemore's abilities, it is also very enjoyable.
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 a 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 initially described as a "cold fish" by Parson. She is quite effective and provided Parson with a lot of useful information about thinkamacy. Their working relationship progresses eventually, and she grows to develop such respect for his abilities that she questions Stanley's decision to replace Parson as chief warlord of GK.
She finds a kindred spirit in Sizemore after their link, and the two appear to be quite, perhaps too friendly with each other.
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.
Real World References
Maggie is most likely a reference to Margaret Thatcher, to whom she bears some resemblance.
Her outfit, specifically the black blazer with white piping, is also surprisingly reminiscent of Number 6 from The Prisoner - a rebellious former spy who refuses to bend to his captors' mind control techniques. (Given that this is a puzzling association for Maggie's character, this resemblance may be unintentional, or hint at an unrevealed backstory.) Maggie is one of the few Goblin Knob units who does not wear medieval-inspired clothing.