Puniru is the central character of the anime Puniru is a Kawaii Slime. She is a living slime who came to life seven years before the main events of the story, when she was created by a boy named Kotaro Kawaii during a childhood science experiment; the reason she became alive is never explained. In her earliest form she looked like a small blue penguin wearing a hat, and she spent years living alongside Kotaro as his companion. Her body as a slime is marked by a color palette that is a gradient of magenta and aqua, and she tends to keep those colors in her hair whenever she takes on a new shape. Although she usually appears as a cute girl, Puniru is genderless. Her birthday is February 29, and she is eight years old by the middle of the series.

Puniru is highly confident and takes great pride in being a shape-shifting slime, sometimes feeling superior to humans because of that ability. She considers herself the cutest being and loves being called cute, which leads her to constantly seek praise. Her behavior is playful, carefree, and mischievous, but she does not truly understand romance or the differences between boys and girls. External factors can affect her, as seen when she consumes iron filings from a school laboratory while someone uses a magnet on her, which makes her extremely naughty for a time.

Her main motivation is to have Kotaro acknowledge her cuteness and openly call her cute. As he grew older and began paying less attention to her, she decided to transform into a beautiful human girl in order to regain his notice. Even after gaining that form, she continues to work toward his approval, which becomes complicated because Kotaro has romantic feelings for his upperclassman Mami Kirara.

Within the story, Puniru is the main heroine and mascot figure. She lives with Kotaro and acts as his best friend, roommate, and little sister figure. Her ability to change into a gorgeous girl and many other forms drives much of the comedy and romantic tension, and her attachment to Kotaro makes her the emotional center of the series.

Her most important relationship is with Kotaro, her creator and closest companion. She wants him to recognize her as cute and is unsettled by his affection for Mami Kirara, whom she treats as a rival for his attention. She also interacts with Alice Okanega, a classmate who has her own feelings for Kotaro, which adds further complications. Other classmates, such as Yusuke Nanpa, appear in comedic episodes and react to her transformations.

Over the course of the story, Puniru develops from a pet-like penguin slime into a more complex figure who begins to navigate what it means to be close to a human as both of them grow up. Her original penguin form now serves as her side bag, where she keeps ingredients that allow her to produce more slime whenever she needs to transform. She tends to melt when she becomes emotionally overwhelmed, and her gradually deepening feelings form part of her growth.

Her most notable ability is nearly unlimited shapeshifting. She can morph into many so-called cute forms, including a beautiful girl, a gyaru style, small animals, a prince-like figure, an elementary schooler, and numerous other appearances, with each form reflecting her desire to be seen as cute. She can also generate additional slime from the materials she carries, and she keeps her characteristic color palette across all of her transformations. While her body and personality are normally stable, outside substances can alter both her form and her behavior.