Gave it a go...
Hello StepVibes,
We appreciate the video you did on Belly, to answer your question about why we didn't materialize the monster in the background is that we had somes rules to respect, like you can see it is a SHMUP experience which is a gender of game that rely a lot on the Highscore. Therefore we decided to create an endless game in which you can always score points as long as you stay alive. Having a boss here would put an end to the game which would lead to a restriction of what score you could do at maximum.
KudoJr