Sega seems to add Hatsune Miku into whatever games they can, but the Vocaloid idol plays a key role in the 7th Dragon series. You save Hatsune Miku in 7th Dragon 2020. Hatsune Miku returns for the sequel 7th Dragon 2020-II and at certain points in the game she can sign in the background if you select "Diva" for BGM mode.
Diva mode has music from 10 artists: kiichi, keeno, koyori, SHIKI, take, nak-ami, Clean Tears, Lemm, millstones, and Wonderlandica. The menu screen also confirms 7th Dragon 2020-II will have a data install option to speed up load times.
7th Dragon 2020-II is scheduled to come out in April for PSP.