Am I a boy or girl?

No. The player isn’t Frisk, they’re different. The player is the person behind the screen, controlling Frisk. Frisk just is the default name for the human. Frisk has no actual personality traits or interests. You can’t tell anything about them. All of “Frisk’s” actions in the game are the player’s actions. Frisk doesn’t spare or kill anyone, that’s your own decision.