Mrs. Rat

Mrs. Rat is a spider character in  that can be found inside a box, inside the Mr. Rat crawl space.

Upon helping the couple, they shall give you Rat Bond blueprints.

Rat Bond Blueprint
In order to unlock the blueprint you must help Mr. Rat by freeing (and not killing because he will be passive) him from a random box in Goldmines levels 1-3 after helping him you will find a random crawlspace again in Goldmines levels 1-3, you will find him alone with a box. Inside the box is Mrs. Rat, destroy the box to get her out and they will give you the blueprint.