My Girlfriend Cheated on Me, and Now My Flirty Underclassman Won't Leave Me Alone (Manga) Vol. 1

When Hasegawa Yuuta discovers that his girlfriend has been cheating on him just days before Christmas, the holidays are looking to be a lonely time for the newly single university student. But when he crashes into his mischievous underclassman, Shinohara Mayu, dressed in a sexy Santa outfit, his life is thrown for a loop. How can he grumble about his broken heart when a cute girl insists on invading his personal space?