A slime girl in a slime girl tf (transformation) story is often a fictional character. She is typically made of slime or has slime - like properties. Usually, she can change her form or interact with the environment in unique ways due to her slime nature.
Slimes in these novels are often depicted as having a gelatinous, amorphous form. They can usually change shape to some extent, which gives them an advantage in various situations, like squeezing through small spaces.