There are a number of unlockables after satisfying game conditions. These can be unlocked by continuing, and I'm not sure if you need to finish the game using the same character throughout but it's the safest choice.

Effect	How to unlock
5 credits	Play for 60 minutes total
8 credits	Play for 180 minutes total
Arcade Mode ++	Complete Arcade Mode
Challenge Mode	Complete Arcade Mode
Challenge Mode ++	Complete Arcade Mode
Character - R.A.D	Complete Arcade Mode using Ruby
Character - Sensei	Complete Arcade Mode using Tempest
Extra Credits	As gameplay time elapses. 5 credits at 60 minutes
Female System Voice	Complete Arcade Mode using R.A.D
Retro Mode	Complete Arcade Mode using Sensei
Unlimited credits	Play for 300 minutes total