Even If This Love Disappears Tonight

When Tooru Kamiya is pushed into falsely confessing his love to Maori Hino, she tells him she’ll date him on three conditions.
1) Don’t talk to her until after school.
2) Keep any communication between them concise.
3) Don’t really fall in love.
Unfortunately, he isn’t able to keep the last rule—and when he tells her how he feels, she reveals that she has an illness that prevents her from remembering anything that happened the previous day, and that she uses a diary to keep track. But Tooru is determined to build a relationship with her, one day at a time…

[From Yen Press]