Intriguing story with lots of potential but... terribly executed.
First of let's start with the Terminology that is essential to Pet.
Valley: The bad memories from ones past.
Peak: As the name suggests, a thought/memory that embodies paradise.
Lock: Disguising your peak in order for it to not be crushed. There are "weaker" and "stronger" locks, the only difference being; that the latter seems to impair the speed at which you can jump realms.
Crushers: Beings that manipulate others peak, in order to alter their memories
and If wanted completely destroy the given individuals personality, or turn them into "babies".
Babies: Mindless pawns that have been crushed into doing anything you please.
Pet: Once someone else shares their peak with you, it automatically renders
you his "Pet" since you both share the same Peak and both Individuals
can be present there at the same time; apparently that "Pet" becomes sexually attached to the sharer of the Peak, at least so It might seem...
Back Door: It acts kind of like a checkpoint/mark, so far it has only been shown that water seems to be able to create puddles as back-doors, which gives access to wherever that puddle is and therefore unobstructed accessibility to that given realm. It should be imperative that any image could create such a Back Door, but that's left for speculation.
Image Users: Beings that can enter other peoples minds to alter their memories in order to make them anything they want them to be, within that memory that they enter, they manifest themselves through a certain "Avatar", It can be a door/fish/water/wind, it hasn't been made clear yet what the given properties are that decide the Image User's Avatar, however it seems arbitrary, although I speculate it depends on the characteristics of the User.
Story: It all takes place in modern day Japan, where it follows the narrative of a certain company that has image users as employees that basically go around crushing/altering/manipulating others through mind-control for the ideals of the organization, and once someone oversteps their boundaries they in return become the collectives enemy and it all turns into a huge deathmatch.
Two cents: The genesis or why there are image users hasn't been mentioned, and it seems terribly nonsensical that beings with the power to mind-control others would subordinate themselves to a petty company for their trivial selfish goals; when in-fact you could do that all by yourself...
I have not read the manga, and this is written from the standpoint of 6 episodes. It should be noted that I have dropped the anime, this will not be updated.