The Monster was first chronologically seen by El: instinct strongly urged her to stay away from the monster. Dr. Brenner ignored her fears and pushed El to make contact and was indirectly responsible for creating a way for the monster to enter the real world.
The Monster is strongly attracted to blood: when Barbara bled into a pool, the Monster quickly took her into its world and killed her: Nancy and Jonathan later exploited that to ambush it. Despite shooting it, beating it and even torching it, the beast survived and slipped away: when it intended to eat Dustin, Lucas and Mike, El used her powers to rip apart the Monster, but somehow also disintegrated herself in the process.
- It's listed in IMDb as simply "The Monster"