Mysterious Girlfriend X is a high school romance between average nice guy Tsubaki Akira and the titular class weirdo, Urabe Mikoto. This series immediately sets itself apart from other romcoms by eschewing the typical Will They Or Wont They
plot in favor of hooking the two up by the end of the first chapter, and focusing on their development and trials as a couple and the sexual tension between them. In typical gender roles, Tsubaki is eager to progress their sexual relationship and Urabe is adamant in taking it slow, but again the series sets itself apart by choosing unusual battles. Tsubaki's early frustrations include Urabe refusing to hold his hand or smile for a photo, but in the same time frame she once strips naked and has him taste her saliva.
This, of course, is the most likely deal breaker for most potential readers. Tsubaki and Urabe first come together after Tsubaki tastes some drool Urabe left on her desk while sleeping after class. It's soon revealed that they can experience each other's feelings by exchanging saliva, with more characters getting in on the act over time. Don't be too off put by the squicky surface, though; after all, it's really no worse than an indirect kiss.
The main cast is kept small, and the characters are well developed. Tsubaki, most importantly, is a very likable protagonist; an average student who truly loves his girlfriend and struggles to balance his own desires with her quirks, sometimes screws up, but recognizes when he does and tries to do better, while Urabe is delightfully strange and impenetrable. Beta Couple
Ueno and Oka provide foils to the pair as both individuals and couples, and are fun to have around in their own rights. Comedy often comes from the boys' frustrations, while conflict comes from sexual tension and romantic rivals.
The series also sets itself apart by its art style. Character designs are simple and expressive without going overboard, blending innocence with eroticism in a perfect mix for the series' tone and subject, and small details of the background and clothing are lovingly rendered. For icing on this cheesecake, the fanservice is very tasteful. The whole thing is a pleasure to look at.
This series is funny, genuinely sweet, and steamy in unconventional ways. I absolutely recommend it, and hope that you'll look past the initial squick to find that you love it, too.