Ruth is
Chise's familiar, a black grim who was originally a church guard dog who chased away grave robbers and evil spirits. He thinks of himself as a human, thus able to maintain human form.
Ruth's original name was Ulysse given to him by his owner (who he also thought was his sister), Isabelle. Isabelle was killed by an oncoming carriage. At the funeral, Ruth believed she would awake and stayed by her grave waiting until he died.
He became a black grim, a fae who chased grave robbers away, however he continued believing he was human up until his meeting with Chise.
As Chise's familiar, he shares a mental link with her, being able to see into her dreams and read her mind. They are still separate entities.
Source: Wikipedia, edited.
