Sean's also the only one who seems to be ENJOYING all of this. Aside from the player, I guess.
Plus there's the contact lenses thing. I think it's that Sean's wearing one so that s/he has two blue eyes, and that the goth friend was sort of a red herring.
From what I'm reading from the diary pages I have so far, though, this is a pretty messed up story.
Chloe checks herself into the hospital due to her being depressed over the player character's moving away. She becomes 'friends' with the patient recently moved into the cell next to her. The other patient expresses that they wish to become 'normal'. As time passes, Chloe and her new friend start to mimic one another as a game to pass the time. Eventually, they're indistinguishable from one another, unless they drop the act intentionally.
The other patient somehow eventually switches cells with Chloe, and is dismissed from the hospital given her depression was 'cured.' Chloe became trapped in her friend's cell.
Between the time span of these events, however, was also that Chloe's friend had written letters to be given to the other patients of the hospital. These letters somehow caused the other patients to become violent, resulting in their attacking the staff. It's possible that this mania was infectious, and that this is why the hospital staff are as they are when the player explores the place.
However means through which they were infected, though, meant that they were unable to fully perform their duties as staff members--they likely stopped feeding the patients, and instead started performing bizarre/unethical 'treatments' on any individuals that they could find. Chloe is eventually caught for this, and afterwards seems to die of starvation.
The player character and their friends seem to arrive some time after all of these events, including among them the person who may be the impostor. This person seems to be somehow luring the other friends to the staff--who eventually "treat" the unwilling person--while treating the entire thing as a game.
Worse, Chloe seems to have been in love with the player character, and kept this attachment past even death. She related her current state before beginning treatment at the hospital to 'heartbreak,' and her final moments were spent calling out to the player character to come help them. Post-death, she still aids the player character and their friends in their escape.
I'm still missing like eight or so pages, and consequentially I haven't seen the third ending. I hope Chloe gets a happier ending than just successfully helping the player character escape. NPCs like Jane Doe seem to imply that she could be sort-of okay, even as a ghost.