Nazo no Kanojo X

is a manga written and illustrated by Riichi Ueshiba. It was originally published as a one-shot story in 2004. It was serialized in Afternoon from August 25, 2004 to September 25, 2014. It is licensed in North America by Viz Media, who serialized it in Vertical. An anime adaptation by Hoods Entertainment aired in Japan from April 7, 2012 to June 30, 2012. An original video animation was released on August 23, 2012.

Plot
The series follows the romantic but unusual relationship of Akira Tsubaki and Mikoto Urabe. Urabe is a mysterious female transfer student who recently came to Tsubaki's high school. After a series of strange events, Tsubaki finds himself addicted to Urabe's drool. She claims the addiction to be "love sickness", and their relationship slowly progresses, focusing on the odd friendship and sometimes erotic behavior that comes out of their drool attachment.

The plot covers the various relationship developments for the couple, and how they're able to deal with, or overcome them via their special bond. Somehow, the drool allows them to experience what each other are thinking or feeling, and allows for a better understanding of each other, more so than any other couple. However, because Tsubaki has never had a girlfriend, and Urabe has never had a boyfriend, and because they both declare to each other that they are virgins, they do like to take things very slowly, with intimate aspects of their relationship being experienced very carefully, many times by trial and error. This aspect, among many other quirky happenings, leaves the reader or viewer wondering when, or if, they will ever kiss.

Nevertheless, as the story progresses, the relationship between the two deepens into genuine, pure love. The manga strongly foreshadows their souls becoming deeply intertwined, and that they will never break up, but rather will stay together and be committed to one another for life.

Official

 * Official Japanese MGX Website

Other Sources

 * Wikipedia
 * Anime News Network (Anime)
 * Anime News Network (Manga)
 * w:c:MyAnimeListMGX
 * Anime Critic's Review of MGX---https://www.youtube.com/watch?v=UBh2NCDMV-c

Other Wikis

 * AnimeBaths
 * Other Romance Themed Wikis---https://romantic-siblings.wikia.com/wiki/Romantic_Siblings_Wiki