Fiction writers use the concept of a backstory to describe what has happened to a character before the plot begins. They imagine relationships, character shaping events, and even locations, to build the persona of the character they are writing about. The fictional backstory becomes part of the novel, or it might just help the writer make the character seem more real while they tell the story.
Your mascot is a real, moving, three-dimensional character, and you can enhance your mascot's personality by creating a backstory. Having a great backstory will add depth and motivation. It makes your mascot's behaviour more consistent, and gives your performer a context to draw from when interacting with fans. Is your mascot shy in front of a young audience or the life of the party? Perhaps your mascot is always curious and this leads to comic interactions?