Mr. Rat

Mr. Rat is a character in . He can be found in Goldmines 1 - Goldmines 3 in a random Crawl Space.

Mr. Rat will appear as an actual rat mob with a box that's shaking, containing Mrs. Rat.

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

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.

Mr. Rat and kill counters
Freeing Mr. Rat from a random box makes him neutral at first, so killing him won't increase any kill counter (e.g. Lesser Demon, Relic Eater, Intensifier). However exiting the room and coming back again makes him hostile, so he is treated as a regular rat and properly increases counters.