The Shadow, usually referred as Shadow Yuuko ( 影 夕子, Kage Yuuko) is a mysterious spirit who resembles Yuuko Kanoe, it's existence and details are one of the biggest mystery of the series. The first character to become aware of it's existence was Kirie Kanoe  , as Yuuko herself seems to have forgotten about her.
Appearance and Personality
At first glance it looks like nothing but a rotten, hateful spirit which somewhat resembles Yuuko Kanoe, the spirit in question describes itself as "ugly" and "dirty". Unlike Yuuko, Shadow Yuuko's appearance is not "set", but actually slightly changes from time to time; however rather than an attribute of "Shadow" this merely seems to be Maybe's artistic sense at work. During the Shadow Yuuko arc "Shadow" had the chance to express more feelings instead than those of her grudge, at those points the "evil" in her figure was toned down and her appearance became much closer to that of Kanoe Yuuko.
Kirie theorizes that the Shadow is nothing but a representation of Yuuko's past grudge and feelings, eventually it is revealed that said statement is the closest one to the truth as Shadow Yuuko herself reveals that she is also "Yuuko" and holds all of the memories that regular Yuuko lost along with her dying wishes.
Shadow Yuuko can take memories and feelings from Yuuko as long as she wants it to, and can return them to her as she wants it as well; Shadow's ultimate objective seems to return everything that she has back to Yuuko and to fulfill her "Wish". Shadow Yuuko seems to be uninterested in interacting or being seen by anyone else other than Kanoe Yuuko. However -by choice- she has revealed herself to some of the members of the cast at some points  ; probably this is so as she believes that this will catalyze Yuuko to face her existence (which at one points, Yuuko also "forgets").
Kirie catches a glimse of her at the end of this mystery.
In this chapter Shadow decides to confront the cast, forcing them to acknowledge her own existence.
Shadow Yuuko appears constantly, always looking for Kanoe Yuuko, at the end of the arc Shadow warns Yuuko as not to repeat the same mistakes that she has already, after seeing their situation mirrored in Kirishima Yuuko.
Shadow Yuuko Arc
Being the central point of the arc, Shadow Yuuko has several appearances and has a hand in both taking and then giving back memories to Yuuko; in the end as she partially fused with Yuuko, Shadow even warned her that at one day Yuuko will have to face her own past.
- At time it's possible to see that Shadow Yuuko has her left leg broken, just like the skeleton in the Paranormal Investigation Club
- When Niiya found Yuuko, he did so because Yuuko called his name out; at one point when reminiscing Niiya mentions this to Yuuko, however she denies this fact saying that he found her on his own. Niiya eventually theorizes that it was Shadow Yuuko the one that called out to him and made him turn around, forcing him to see Yuuko for the first time.
- "At last, you found me" — As Teichi and Kirie became able to see her.
- "I... really loved you" — As she explains Yuuko's situation to Teichi, chapter 18