I don’t think that’s a manga....they r voidloids...I think that’s how I spell em
Vocaloids
Not a manga, xD It's Vocaloids. Robots (kinda, I don't know the right word for it) who sing.
There are mangas with the people present but yes they are mostly computer synthesized and made-up-mascot-for-the-voices singers. Names such as Hatsune Miku, Kaito, Len and Lin Kagamine, etc. Mangas would have a name like "Hatsune Mix" or something similar. If you like their designs there are music videos of them dancing and singing and stuff.
What Manga is this one http://www.mangago.me/read-manga/i_have_a_boyfriend/nbt/c003/pg-27/