ばいばい、ノスタルジーカ (Bye Bye, Nostalgica)

Background
If I remember correctly, the girl's name is Nostalgica. I can't recall anything more than that. "Bye Bye, Nostalgica" is an original song by Neru featuring Kagamine Len.

This song has entered Hall of Fame. It is featured in his album My Name is Love Song.