In Inside Out, the emotion that is always sad is Sadness.
Sadness is one of the five personified emotions guiding Riley Andersen, alongside Joy, Fear, Disgust, and Anger. Voiced by Phyllis Smith, Sadness is blue, wears a cream-colored turtleneck and glasses, and initially seems to Joy like an obstacle to Riley's happiness.
The film gradually reveals that Sadness has an essential purpose. She helps Riley recognize when she is overwhelmed and encourages other people to offer comfort. A memory that begins sadly can become meaningful and even partly joyful when Riley receives support from her parents and friends.
A common mix-up is assuming that Joy is the film's only important emotion because she leads the other characters. In fact, the story's central lesson is that emotional health requires more than constant happiness. By the end, Sadness helps Riley honestly express that she misses Minnesota, allowing her parents to understand and comfort her.