Genre:
Romantic comedy

Mysterious Girlfriend X (謎の彼女X, Nazo no Kanojo Ekkusu) is a Japanese manga written and illustrated by Riichi Ueshiba. It was originally published as a one-shot story in 2004 before becoming a serialized comic in Kodansha's Monthly Afternoon magazine in 2006. The manga is licensed in North America by Vertical. An anime adaptation by Hoods Entertainment aired in Japan from April 7, 2012 to June 30, 2012,with an original video animation episode released on August 23, 2012.

Plot:
The series follows the relationship of Akira Tsubaki and Mikoto Urabe. Urabe is a transfer student who recently came to Tsubaki's school. After a series of strange events, Tsubaki finds himself addicted to Urabe's drool. Once she claims the addiction as love sickness, the relationship slowly progresses, focusing on the odd bond that comes out of the drool attachment.

As the story progresses, the relationship between the two deepens into genuine love. The manga strongly foreshadows that they will never break up, but rather will stay together for life

Review:
This anime was really strange  disturbing but at same time it was really fascinating, i think the anime ended on cliffhanger, I can't remember much about it sadly, since I watched it back in middle school.

I'd say watch it, let me know what your thoughts about it.