Mrs. Ko was Mitchell Pritchett and Cameron Tucker's landlady.
In "Slow Down Your Neighbors", while Cameron was taking the trash out, he encountered with Mrs. Ko and told him that she had just finished showing their apartment upstairs. Cameron then mentioned that his new friend, Barry was living there, but, however, Mrs. Ko told him that the apartment was not taken over by anyone (she didn't even know who Barry was). This surprised Cameron, who later realized that Barry had been living in Lily's princess castle, as he was homeless.
It was revealed in a deleted scene from "Best Men", that she died in the past two years.