If you're looking for manga similar to Mysterious Girlfriend X, you might like these titles. All recommendations are made by Anime-Planet users like you!
Koharu no Hibi and Mysterious Girlfriend X both feature, umm... mysterious girlfriends!? So if you found the theme interesting in one of those two manga, you could give the other one a try. Just keep in mind that unlike Urabe (who is bizarre at best), Koharu can be pretty scary and disturbing...
For the last year, Ryouta Kusahana has lived a 'typical' high school life – except for the fact that he refuses to take off a giant dog suit. The only classmate who shows him kindness is class rep Hinako, a girl that Ryouta has fallen in love with. So when the new trainee teacher Ayame catches the boy sniffing Hinako's gym shorts one day after class, Ryouta is devastated and pleads for her to keep it a secret. But Ayame demands a high price: in exchange, Ryouta must let her fulfill her perverse fetish to smell and sniff him, as she's fallen in love with his scent at first sight. Now, though he wants nothing more than to win Hinako's heart, Ryouta must come to terms with this new woman who's wriggling her way into his suit, and all over his body.