The Serious Red Flag (Honki no Shirushi)

Tsuji is a young salaryman who finds social situations incredibly boring and unpleasant. Due to his masterful pretense of friendliness, he has friends among his coworkers and even two girlfriends. One night, he meets the enigmatic Hayama Ukiyo at a convenience store and learns that sometimes being helpful can cost him dearly. After their meeting, he can't quite get Ukiyo out of his head, even though he knows she should mean nothing to him. Against his better judgment, he finds himself being pulled further into her crazy world.