Default Neco Arc voice is done by Yuzuki Ryouka (Arcueid). To unlock extra Neco Arc voices done by other voice actors (selectable in the Special menu), repeatedly clear the Survival mode (meaning win 22 matches and go through the entire roster of playable characters) with Neco Arc.

Unlockable	How to unlock
Hitomi (Akiha)	Clear Survival mode the second time.
Kouzuki Miwa (Miyako)	Clear Survival mode the third time.
Mizuhashi Kaori (Len)	Clear Survival mode the fifth time.
Natsuki Rio (Sion)	Clear Survival mode the fourth time.
Sakuma Kumi (Ciel)	Clear Survival mode the first time.