At the end of "Arden and the Girl Downstairs" by Amber Davis, the mystery unravels as Arden discovers that the girl living downstairs, Lily, is actually a ghost. Arden learns that Lily had passed away years ago and was stuck in the apartment, unable to move on. With understanding and compassion, Arden helps Lily find closure and say goodbye to her past, allowing her to finally move on peacefully. The story ends on a bittersweet note, as Arden's kindness brings a sense of resolution to both their lives.