Complex backgrounds removal/selection issue

The whole point of tools like Remove Background is that they use Ai to work out what to keep and what to mask.  So it would identify a human shape or a building, for instance, and I have never seen a human that looked like that (so far).  

I would do pretty much what Kevin suggested, and select color blocks, but in this case I would delete rather than mask because it is going to be much quicker.

With a drawing (copyrighted btw) this complex, you could spend more time trying to find an automatic/one button solution for a drawing (not a photograph) to remove the background.

Just go old-school and start selecting with the Quick Select tool. You have an advantage that the Jerrick figure is outlined in black giving you a solid frame to exclude.