This unnamed Baby Mouse is Fitz's female child and daughter. The mouse is a minor character in 12 Oz. Mouse, but she is part of Fitz's family. The baby is known as a she, due to Fitz asking a hand of an unknown Doctor, "Is she supposed to be that small?" She is seen in Mouse's flashbacks, along with her mother.
The Baby Mouse is seen on the beach with her mother and father. She also has a crib, and cries in it, which her mother tells her to hush, as The Shadowy Figure appears in the white doorway.
During an another flashback sequence, she cries in her crib when Fitz goes to her until she later morphs into a giant Spider, possibly due to the effects of arachno-xenomorphentation. Although an another Spider character does appear in the show, which she has same appearance of, so it's unknown if this is the same spider or not.
The Baby Mouse has a similar design as both her mother and father, but is small, and pale, has stick-finger arms, legs, a nose, whiskers, one eye, a mouth, small ears, and a very small tail.