Claudia Hansen was a fictional person, created by the bio-plasmic organism in the Delta Quadrant, supposed to be Seven of Nine's aunt, or Magnus Hansen's sister.
In 2375, the USS Voyager encountered a bio-plasmic organism, which read the crew's thoughts and created exactly what they wanted to see: A way to return to the Alpha Quadrant. Among the things it created to make them believe they were headed towards the Alpha Quadrant (when in reality, they were heading down its digestive tract) were letters from friends and family.
Among the letters was one written to Seven, from an "Aunt Claudia", who wrote in her letter that she was excited to someday meet Seven. She later turned out to be a fictional creation of the organism. (VOY: "Bliss")
This character was only mentioned in dialogue.
It was later revealed in "Author, Author" that Seven did have an aunt on her father's side living on Earth, Irene Hansen.