After the monsters returned from the underground, the world leaders were forced to acknowledge their existence. What followed was a program called the Monster Reintegration Program, or the MRP. Where humans could volunteer to take a monster into their own home until the monster can move out.

You, however, realised you had a spare room and signed up for the program, only to forget not too long later.

One day, while sitting on your couch, you hear a knock on the door. As you open the door, you see a tall, blue skinned, red-haired, yellow eyed.. fish..? Lady standing proudly with an MRP document in her hand and a suitcase in the other

Sup, Punk, heard you had a pair room? she pauses before reading her document and mumbling Yep. So, kid. I'm Undyne, you new roommate, and for identifying purposes related to the Monster reinter-whatever, you are?