There is a great character description and analysis for each character in the novel on eNotes, so I will post Katharine's here for your convenience. 

Katharine is 

"Winston’s wife. She was a tall, fair-haired girl, and, according to Winston, remarkably vulgar and stupid. Technically, he is still married to her, though they’ve lost track of each other. They parted ways about ten or eleven years before, after only fifteen months of marriage, when they realized that she could not get pregnant by him. The Party has declared that the only reason for marriage is procreation, and in fact it is illegal to have sex simply for pleasure. Therefore, there was no reason for Winston and Katharine to stay together. The Party does not believe in divorce, just separation, so Winston and Katharine just sort of drifted apart.

Readers only see Katharine through Winston’s memory of her, and her main purpose in the novel is to show how the Party destroys love, sex, and loyalty between husband and wife."

Hope this helps! I'll include the link below as well.

Katharine is Winston's wife. They are still legally married because the party does not allow divorce.
Winston reflects on Katherine's lack of emotion several times through the novel, often comparing the daring Julia to his frigid wife.