Nazo no Kanojo X  

Mysterious Girlfriend X, 謎の彼女Ｘ

Akira Tsubaki sat next to Mikoto Urabe in high school classes. On her first day, Urabe was known as an oddball due to her sudden burst of laughter during a lesson without any apparent reason. She preferred not to interact with others and often slept on her desk. One afternoon, Tsubaki discovered Urabe napping at her desk and woke her up. Later that night, he tasted the drool on her desk and became ill. A few days later, he was visited by Urabe who confirmed that his fever was caused by "love sickness" resulting from withdrawal symptoms of her drool. As a result, the two developed an unusual bond where Urabe provided Tsubaki with her drool daily to prevent further illness.

Nazo no Kanojo X was published in English as Mysterious Girlfriend X by Vertical Inc. under the Vertical Comics imprint in 2-in-1 omnibuses from March 15, 2016 to June 13, 2017. The series was digitally published in English by Crunchyroll.