In canon, Mistmouse is described as a light brown tabby she-cat. She was one of the last tunnelers (hence the big paws!) and a descendent of Windstar.

She was a tunneler back when they existed, mostly seen in Tallstar's Revenge. She's a mate of Hareflight and a mother to Stagleap, Ryestalk and Doespring. Despite being a tunneler herself, she specifically wanted her kits to be moor runners to protect them from the dangers of the tunnels. She was nice and helpful to Tallpaw back when he first started his apprenticeship, and seemed to be a friend of his father, Sandgorse. She's mentioned with Plumclaw and Woollytail a few times too, so I always saw them as friends as well.

She's said to be a direct descendant of Windstar, making quite a few of her own descendants, like Onestar, Crowfeather or Gorsepaw, Windstar's distant kin as well.