Always helpful to know just how to unlock the extra characters and modes in this game to help with achievements, so I'll list what I've found so far.
5 credits - Play for 60 minutes total
8 credits - play for 180 minutes total
Free play - play for 300 minutes total.
R.A.D. - Beat the game in arcade mode using Ruby
Sensei - Beat the game in arcade mode using Tempest
Challenge mode - beat the respective level in arcade mode
Arcade++ - Beat the game once
Challenge++ - Beat the game once
Retro Mode - Beat Arcade mode with Sensei
Female AI Voice - Beat Arcade mode using R.A.D.
If you have trouble beating the game with a certain character to unlock one you don't have yet, play with a character you're good with until the last boss, save one continue and decrease it's HP until close to death. Die and pick the character you need to beat the game with, finish off the boss and enjoy your unlocked extra character!
Thine eyes fool half deceiving, world of illusions feeding delusions, open thine mind, open thy soul, take in truth, cast out lies, see everyone, see everything for what truth really holds.
Kitsune love video games, nothing wrong with that one bit! Have trouble with a game? Just ask for some vulpine luck and intuition to guide your way!