Hatsune Miku is a Vocaloid (Which is a voice synthesizer that was given a personality and overall look) by Yamaha, and she is one of my favorite divas EVER. I love every installment of her game and have played most of them if not all of them. It's rather impossible for me to go without a year and not cosplay any of her versions.
(Which is I tell you, pretty countless).
About the costume:
It's a modified version of their Project Diva uniforms. As the freedom to add accessories and huge neck ribbons was implemented during Dreamy Theatre and Project Diva F, so I took the liberty of personalizing her to some extent. I made the costume hours before the convention. (In which case, ironically I actually made 2 other costumes and finished everything. But ended up wearing Miku