Kono Kanojo wa Fiction desu.

Lead character is a naive boy who's fallen in love with an older girl who everyone considers a genius writer. He's so in love in fact, that he has forgotten about his secret hobby for the last 2 months. His secret hobby is that for the past 10 years he has been writing the "character profile" of a single girl. Her birthdate, personality, hobbies, conversations, etc.... is all recorded in a large pile of notebooks. All this time this fictional girl has been his friend and lover, but now that he is in love with a real girl he decided to throw them all away. In order to "purify" the notebooks, he decided to burn them at a nearby shrine. To his surprise, the sempai he's in love with was also there. It starts to rain and as they talk the girl confesses she's having trouble writing her novel due to inexperience. As the storm gets even stronger, the lead character believes that since she confessed her "weakness" to him it is ok for him to show her his own "secret". Filled with courage, the boy opens the bag where he brought all the notebooks... except that instead of notebooks, the bag now contains the girl (naked of course) he's been creating for the past 10 years!!
(Source: Baka-Updates Manga)