The Protagonist is the main character of Devil Daggers and is the character controlled by the player. Nothing about their appearance, voice, or personality is known except for the appearance of their hand.
All that is known is that they came across the Dagger. They were then teleported to the Arena and forced to fight the enemies until death.
It is probably very painful for the Protagonist to use the devil daggers when it's in a 3rd state (after collecting 70 gems). The idea on this is coming from the fact that while shooting, the Protagonist is screaming. His body is most likely under huge stress because of being modified by gems and (of course assuming that Protagonist is a human) having to create this daggers from (probably) his own flesh, making him feel how he's ripping his own body to materialize daggers with which he can fight