Who is Okusan from Kokoro and what is their importance?

Okusan is a widow with a young daughter. She is looking for a boarder when she meets Sensei, and she offers for him to move in right away. Okusan appears to be a silent observer of the happenings in her home, only stepping in if she feels situations are becoming too personal. She admires Sensei, and she tells him often that she believes him to be studious and generous. Relying on the same instincts that prompted her to welcome Sensei, she refuses K's admittance to her home at first. She later gives into Sensei's demands, and she also warn him that he will regret his decision.