A.K.A. "Mary Poppins Syndrome". An easy way to add anywhere from a twinge of sentimentality to a whole extra Tear Jerker scene to the end of your movie: after helping out and saving the day, The Drifter character must leave forever. Used a lot in solo series of many an Ensemble Darkhorse, as their adventures alone can't keep them in the small town forever; they have to plausibly be able to get back to the team. This tends to happen a lot in Girls Love anime, especially if the heroines don't end up crazy or dead.