Monika always loved you, and at a certain point she reached her breaking point, and deleted the other girls, leaving you with only the option of her, but you will rejected her, having been in love with Yuri. After this Monika finally broke, and any sanity she had left was gone.

The next day she calls you over, and you find her dressed in Yuri's old clothes. Not only that, but she has dyed her hair purple, and is wearing contacts. She has stuffed Yuri's old bra, and is holding onto her notebook, and the novel that she was reading. Her smile is bit too wide.

Monika: "Sempai! H-Hi! It's me, Yuri!"