It tickles me since a while now ... Who's that girl next to Gumi ?

It's the KZR-28 Prototype Singing Robot, which the boy wearing the goggles in the background ends up calling Neon since he found her in the garbage and neither knew her real name. She was created for Crosslight, kz's latest commercial album:

http://www.youtube.com/watch?v=S1SX4S5syoA&feature=player_embedded

Dunno why kz and yuki masami decided to make her though, since she's essentially Miku sung at a pitch that ryo used for Melt. Her fingernail colors give it away.

I see, well, thank you for your explanation, it seems a lot clearer now.