Creator Monty Oum designed the world map by squirting ketchup on a napkin, scrunching it up and then unfurling it. He then used the shapes the ketchup made as inspiration for the size and shape of the land masses.
Blake Belladonna's character was inspired by the character of Belle from "The Beauty and The Beast". This is referenced in her last name Belladonna and also in the "Black" trailer, when she is with a boy named Adam. Adam is the name of the man who became The Beast in Beauty and the Beast (1991).
After Monty Oum passed away, the decision was made to have his brother Neath Oum provide the voice of Ren rather than write out the character or make him silent.
Yang Xiao Long is inspired by the fairy tale character of Goldilocks. While her inspiration may not be quite as obvious as the other three main characters, series creator Monty Oum has said that Yang's personality is to jump straight into a situation without thinking, which relates to Goldilocks' story where she enters a strange house to eat food and sleep in a bed without thinking about what would happen when the owners returned.
Weiss Schnee's character was inspired by fairy tale character Snow White. This was referenced by the song she sang during the "White" trailer, called "Mirror Mirror". Her full name also translates from German to "White Snow".