Nazo no Kanojo X

Synopsis

A strange new transfer student named Mikoto Urabe is instantly labelled as a weirdo due to her odd sleeping patterns and a strange outburst of laughter during class. When Akira Tsubaki wakes up Urabe at the end of a school day, he finds her face enchanting and ends up sampling some of her drool. After having a dream about Urabe that he can't seem to get out of his head, Tsubaki suddenly comes down with fever. Urabe goes to Tsubaki's house and gives him more of her drool which cures his fever, explaining his symptoms were the result of withdrawal from not getting any more drool. As such, Urabe begins meeting up with Tsubaki every day to give him some more of her drool. As Tsubaki starts to fall in love with Urabe, he decides to confess to her after a few days. When asked by Urabe to profess his love in a way only he can, Tsubaki demonstrates it by ripping up a photo he had in his wallet of his middle school crush. After explaining the reason she laughed in class was because a voice told her Tsubaki would be the first person she would have sex with, Urabe agrees to become his girlfriend.

(Source: Wikipedia)