I liked the idea of you being invincible but the boy/girl aren't, would have been fascinating to see chicken and eggs as well. On the gameplay side I felt the difficulty is not increasing so it becomes boring aftee you have slain 250-300 enemies. Just thinking more about it you could have made the boy and girl do dumb things that would end them in trouble like drowning inside a lake, climbing a tree etc and you would have to go save them from that as well.
Overall a good idea just needed a little more variety