HOPP, HELP and HURT each have a different goal.

HOPP wants to get to the gem on the far right side, without getting killed
HOPP can run, jump and even wall jump, but has to avoid touching the deadly spikes,
acidic drips, energy bullets or ghost skulls, or getting squashed.

HELP wants to alter the level to help HOPP get to the gem.

HURT wants to alter the level to kill HOPP before they get to the gem
after placing enough pieces, HURT can place a ghost skull that will chase HOPP indefinitely.