She is voiced by Diane Lane.
She is very kind and motherly to Riley, and she helps her family get through tough times, like the big move. She is very understanding. There are times when she isn't patient with her husband because of his oblivious personality (she thinks she shouldn't have turned down her chance to marry a handsome Brazillian copter pilot) but she still loves him. She also loves Riley and does her best to understand Riley, especially as their move negatively affects Riley's personality. Since Sadness seems to be the leader of her emotions (but is not a weeping mess like Riley's Sadness), it is possible this is where her understanding and compassionate nature comes from as sadness represents the ability to empathize with others or that she might've had a rough past.