"Nazo no Kanojo X" starts... Well, both usually and strange. Main character is Tsubaki Akira, a normal second-year high-school boy. He is quite ordinary, without girlfriend, although he would like to have one. At the beginning of the second year, a new transfer student is introduced, Urabe Mikoto, and is placed on seat next to Tsubaki. He initially tries to be friendly to the new girl, but she turns out to be a strange and quiet loner, ignoring Tsubaki and literally everybody else in class, spending all free time sleeping at her seat. After the first shock, Tsubaki gets used to this and she just melts to the background of class. Until one day! Returning suddenly one day to the classroom, Tsubaki finds her sleeping later, and after waking her up, she notices a pool of drool left on her table. Tsubaki, being a horny teenage boy, disturbed by noticing how pretty actually Urabe is, decides to lick her drool. He feels shocked and ashamed by his behavior, but forgets about it soon. Next day he has a strange dream about her, and few days later he gets a fever and has to stay at home. He is surprised when Urabe visits him after school, uncovers his licking of her drool, and tells him that from now on he is addicted to her drool and that they now share a bond....
Well, this is not the strangest premise of romance anime I have ever heard. Just going through my list on AniDB ("Bakuman", "Itazura na Kiss", "Kareshi Kanojo no Jijou", "NHK ni Youkoso!"... And I will not even start with those based on dating-sims) reveals Japanese fondness of bizarre reasons for coupling two persons, but "Nazo no Kanojo X" definitely has one of the more nauseating - drool. But if you are not very squeamish, don't be thwarted by this seemingly disgusting premise, because "Nazo no Kanojo X" is one of better anime I have watched this year and is definitely underrated. Although this sounds like a recipe for ecchi anime, it is not: this is a seinen romantic comedy. There is some fan-service (panties shots, boing-boing), but nothing over the top; girls are sometimes shown naked, but their bodies are without features, only silhouettes. I just realized that this anime is very similar to "Nisemonogatari": even though there is a lot (and in "Nisemonogatari", a really lot) of sexual innuendo, this is not a primary point of this anime. If you are watching either of this anime for fan-service or arousal, you are completely missing their points. There is also a noticeable supernatural streak in here, although it is taken for granted. Any yes, the show justifies its name ("Mysterious Girlfriend"), because there is a lot to reveal yet about Urabe.
"Nazo no Kanojo" is definitely a seinen anime. Although there is a lot of blushing, nosebleeds, and drooling, as you could expect from anime romance comedy, it is the characters that make this a real seinen anime. By this I mean that they are not one-dimensional and perfect: they are bizarre, but they are also realistic. I know that this sounds like two opposites, but if you are a fan of anime, you know what I mean. For example, Harima from "School Rumble" if unrealistic idiot (I don't mean this derisive, he is one of my favorite character ever); Tsubaki on the other hand is quite realistic idiot. He is a naive/innocent/goffy like a teenage boy that he actually is. Urabe, who is on one side completely bizarre character, on the other hand is reasonable and multifaceted character, which is able to recognize the flaws in others and her, but she is also able to accept them as part of people. Tsubaki is the main character and we see the story mostly from his POV and hear his thoughts, but there is lots of Urabe POV's and inner observations. Both of them are strangely shy. Except them, there is only few other characters in series and they are the usual one-dimensional (but funny) anime characters.
Jokes in the anime are good, and they are not of laugh-out-loud type and more focus is put one romance than comedy. And of course, all is all mixed with this strange topic of sharing drool (and not by kissing).
At the beginning, I was almost put off be "old-school" design of characters, something that I connect with older anime like "Akira" or "Golden Boy", but I soon get used to it. "Nazo no Kanojo X" actually has a pretty good animation, design of characters and vivid colors, which is especially seen in Tsubaki's dreams, which are very imaginative and impressive. The show has pretty good sounds and background music. Two other things I appreciate: people wear different clothes and even though at first it looks like Tsubaki lives alone with his sister, it is later shown that this is not true. There are 13 episodes in series and there is an additional OVA, which is actually a regular episode. Manga is still on-going and I hope that there will be a second season some time in future.