Monogatari Series: Second Season EP20 - Shinobu Time, Part Four

In exchange for her help, Gaen requests three tasks from Koyomi, all being connected to Suruga Kanbaru, formerly known as Suruga Gaen. Izuko reveals herself as her aunt, but she doesn't want to be introduced to her as one to spare Suruga's feelings. When Koyomi accepts, she shows the darkness' true motive—it is a manifestation of the natural order, and means to destroy apparitions not behaving as they should—in this case, not Shinobu, but Mayoi. She exposes Mayoi as a "ghost of a ghost," who should have passed on on Mother's Day, when she finally found her way home; she also stopped leading people astray, neglecting her duty as a lost cow. Gaen leaves to allow Koyomi and Mayoi to process this, and Mayoi realizes she has to pass on, rather than be eaten by the darkness. Bidding Koyomi farewell, she kisses him and confesses her love for him before vanishing. Four months later, Koyomi tells the story of the incident to only one person—Ougi Oshino. On his way home, he realizes he never told Mayoi one thing he should have said: goodbye.