This is a "character" in the voice synthesis application suite Vocaloid.
Hatsune Miku (Japanese: 初音ミク), sometimes referred to as Miku Hatsune, is a humanoid persona voiced by a singing synthesizer application developed by Crypton Future Media. Hatsune Miku is a 16-year-old girl with long, turquoise twintails. She uses Yamaha Corporation's Vocaloid 2, Vocaloid 3 and Vocaloid 4 singing synthesizing technologies. She also uses Crypton Future Media's Piapro Studio, a singing synthesizer VSTi Plugin. She was the second Vocaloid sold using the Vocaloid 2 engine (the first being the English Vocaloid Sweet Ann), and the first Japanese Vocaloid to use the Japanese version of the Vocaloid 2 engine. Her voice is modeled from Japanese voice actress Saki Fujita. Hatsune Miku has performed at her concerts onstage as an animated projection (rear cast projection on a specially coated glass screen).
The name of the character comes from merging the Japanese words for first (初, hatsu), sound (音, ne) and future (ミク, miku), thus meaning "the first sound of the future", referring to her position as the first of Crypton's "Character Vocal Series."