Len-kun and The Onee-san [Len's Diary] (mature? story warning) LenKu ship fanfic

Kagamine Len, a 14-year-old amateur singer enrolls into a music school called ONGAKU 39 for future musicians. Hatsune Miku was announced as his assigned Senpai in class and at first, Len was scared of her because she was strict to her Kouhais. But as Len spent more time with her, he gradually realized that Miku has a good side and becomes happy when students behave. That made Len fall in love with the 16-year-old, but will he be able to win her heart regardless of her age? Read to find out!