Mouse
Mice are a group of unseen recurring characters in Undertale. Their holes are scattered in various places throughout the underground.
Mice reside in small holes near SAVE Points. They are often found near some sort of food item. Interacting with the hole prompts a squeak noise.
The mice struggle to access the food item they are situated near. Per each SAVE Point dialogue, they never give up attempting to eat the food, which fills the protagonist with determination.
In the Ruins, a mouse is situated in the Mouse Hole room, preceding Napstablook's room. It contains a SAVE Point on the left and a table with cheese on it to the right. The mouse hole in this area resides closest to the way the protagonist came in. In the True Pacifist Route, the mouse successfully eats some of the cheese.
The next mouse hole is found in in Snowdin's Spaghetti area. Here, Papyrus has left a plate of spaghetti on a table, and an unplugged microwave next to it. On the left there is a note explaining the purpose of the spaghetti. The mouse's hole is located at the top of the screen in the wall that had flowers at the top. In the True Pacifist Route's epilogue, the spaghetti becomes lukewarm, implying that the mouse tried but gave up on trying to eat it.
The next hole hole is located in Waterfall's Crystal room. In this room lies a table with a piece of cheese stuck inside of a pink crystal. To its right is an Echo Flower that echoes the mouse's squeak sound through text. The mouse hole is located in-between the cheese and the flower. In the True Pacifist Route's epilogue, the dialogue reveals that the mouse had managed to free some of the cheese from the crystal.
The last mouse hole is situated in Hotland's Bad Opinion Zone, which contains a table with a safe on it. The mouse hole can be found right next to this table on the right side. The dialogue reveals that the safe has melted the cheese inside, causing it to be stuck to the table. In the True Pacifist Route, the mouse had managed to "hack" the safe and free the cheese inside.