She likes to draw the Marsh house where she spend a lot of time as a child with Marnie. Later on in the story Sayaka found Hisako's painting in her room. At the back of the painting Anna Sasaki read her name. So Anna and Sayaka go visit her to ask about Manrie. Then she revealed Marnie's past.
In Joan G. Robinson's novel, she is called by her pet name Gillie.