She is located in a tunnel near Dave Spector. To meet her, you must kill Dave Spector with the knife and walk into the tunnel from the west. She will be somewhere in the darkness - it is a good idea to use the Cat effect to locate her.
Interacting with her will trigger a brief full-screen event, where a picture of her face will fade in and out upon the screen. After this, she vanishes away to another place in the area.
In YUMENIKKI -DREAM DIARY-, she is seen in a school crying, rather than smiling in the original. You have to collect four scraps of a photograph, assemble it and show it to her. She is next seen in the Staircase, sitting on the stairs next to KyuuKyuu-kun. Approaching her will cause her to scream at you and a jumpscare-y full-screen event takes place.
Monoe could be a vision of when Madotsuki was happy (or how she wants to be happy). She is one of the only characters that smiles and doesn't seem hostile (like Uboa) but then she disappears. Monoe and Madotsuki also share similar appearances (shown by the picture above).
Monoe appears to be a tall girl with extremely pale white skin, black hair that extends over her shoulders a little bit with straight choppy bangs hanging on her forehead. She has black eyes and wears a white dress shirt with short sleeves with her flaps showing. She also wears a black skirt, white ankle-length socks, and black penny loafers.