Mr. and Mrs. Gardiner are Elizabeth's most sensible relatives, even though they have no training in the deadly arts. Hey, you don't have to slay zombies to be awesome in this story, right?
The Gardiners' main purpose is to function as surrogate parents of a sorts for Elizabeth. Hey, someone has to do it. You can read more about their characters from the original here. The only big thing to note is that Mrs. Gardiner does get one minor update in this volume:
Fatigued as [Mrs. Gardiner] had been by the morning's attack, they had no sooner dined than she set off in quest of her former acquaintance, and (unbeknownst to the sleeping Mr. Gardiner) her evening was spent in the satisfactions of intercourse renewed after many years' discontinuance. (43.71)
Very saucy. We're blushing. What would Jane Austen think?