This game is more of a concept rather actual game play, I appreciate you checking this one out as well.
Here's the instructions when you arrive at a gray screen
"Pressing 1 (Callisto) or 2 (Janus) Choose character at beginning and R to restart game."
So the second you see the gray screen, tap on 1 or 2 and you'll bring forth the character and just play test them for a bit. Pressing R will help you soft reset the game so you can try the other character. Unfortunately I only got around doing 2 character's move sets and not finish the game entirely.