It's established that Kevin works at the zoo. We see a locker with his name on it. However, when Kevin briefly regains control, he tells Casey that his last memory was being on a bus on September 18, 2014. This implies that his other personalities have been in control for roughly two years. If so, did any of Kevin's co-workers notice? You'd think it would raise some eyebrows if he came to work with a different personality and insisted on being called "Barry" or "Dennis", for instance. The walkie-talkie incident indicates that the zoo's employees aren't the sharpest knives in the drawer, but even they would notice if someone they worked with was acting completely different.
One of his meetings with his psychiatrist implies Barry was in charge when they were working the entire time, but given how most things work, it's likely they were aware of Kevin's Split Personality disorder, but had been assured by his doctor he was a high-functioning case of it and no danger to anyone (as until the Horde took over, Kevin was harmless and completely capable of functioning in society).
Certain businesses and companies make an effort to hire people with various mental disabilities, giving them simple jobs and sometimes with a sponsor fully aware of their situation so that they can help them in case of an incident. That is the most likely case here, as Dr. Fletcher also seems to be in regular contact with them. We don't know what his exact job was; it's unlikely he was in management or anything with major responsibility. Since Kevin has so many personalities, it is something his co-workers would have to be aware of; "Barry" is normally the dominant personality, and that is probably the person they are most familiar working with. Unless someone monitors him constantly, no one would notice that Kevin hasn't taken the light lately.
Also keep in mind that while we as the film goers are aware of (and it's called attention to) the various quirks, that may not be true for his coworkers. Not everyone necessarily has a close enough relationship with enough coworkers where that would be something they notice. Also since all the personalities have an incentive to either help Kevin maintain a normal life or hide that their intentions, they would do their best to downplay their persona and mimic Kevin or Barry if they could or excuse themselves from the situation (physically or mentally). Depending on the coworker as well, some of them might not mind or even enjoy the company of varying personas because of specific interests, curiousity, or whatever. And, if nothing else, people change — Kevin has been working at the zoo for at least 2 years, probably more, and people do develop habits, interests, and so forth in that time. That wouldn't be particularly unusual for anyone.
Exactly what standard is the Beast using to determine someone to be "pure" or not? The whole point of kidnapping the girls was to use them as sacrifices for the Beast personality on the basis of Dennis assuming they were air-headed teenagers who never suffered in life, and while Casey is the only one shown to have suffered abuse previously, it seems the only way the Beast knew this was because he just happened to notice the literal scars she had at the last minute. The thing is, though, there have been a good many people who have suffered abuse or trauma but don't carry physical scars to show it. Not to mention that being kidnapped and held captive in an unknown location by a guy with multiple personalities (all of whom are saying something horrible is going to happen to them and none of whom are either able or willing to help) would have to be pretty traumatic, especially for said "air-headed teenagers" who had no prior suffering to compare it to. So wouldn't the very act of kidnapping the girls and holding them against their will in a confined space with it being made clear to them that they're going to be horribly murdered count as trauma and thus technically make them "scarred"?
Well, for one, Dennis looking to facilitate his fetish of seeing the girls dance naked was shut down by Patricia, and the idea that they lack trauma is foreshadowed by both what Patricia said and when Dennis apologizes to them. So although they are kidnapped and that is a scary thing to happen to you, the personalities are doing what they can to make the experience not so intense that they are purified. It makes sense in their own twisted logic, especially as we are dealing with metaphysical premises and not actual purity scales.
Hedwig mentions at one point that Dennis had been following the two girls (presumably Claire and Marcia) for a while before kidnapping them, so it's likely in the time spent following and researching them he came to a conclusion to meet his standards of what he deemed "impure". Casey just happened to be in the car when the kidnapping ended up taking place (the only reason she was with them was her uncle failed to pick her up and Claire's father insisted on driving her home rather than her taking the bus), she was never intended to be kidnapped with them, so Dennis hadn't properly deducted whether she was pure or not.
Why does Casey immediately know/assume someone named Hedwig is a boy? 1) She's already seen Patricia, so she knows Kevin has or can have female identities. 2) Hedwig is not just a typically female name; it's an exclusivelyfemale name. 3) Hedwig does not say anything that identifies or even suggests he is male before she starts telling him that Dennis "wanted a boy this time."
Hedwig dresses like a boy. The only known female personality dressed like a woman, so it was a safe assumption that if dressed like a boy, then that personality is male.
Two known female personalities, actually. Other being Jade. I would assume we can discard Jade, though, since she's only seen briefly.
I'll buy the kidnapper having a multiple personality disorder, but gaining super strength and agility as the Beast?! That's kinda hard to swallow if he's supposed to be a human. Could he be a gifted super being like David Dunn? Seems like there is some super natural stuff going on here. Also, he can get shot, but not stabbed?!
Shotguns and elderly women wielding a cheap-looking dinner knife with limited mobility produce different levels of force. Besides which, that two practically point-blank shotgun blasts didn't leave him a bloody mess with large chunks missing says he's still pretty tough, just not...unbreakable.
The movie does explain briefly what was going with the Beast. The psychiatrist lady had a theory that people with split personalities could slightly change their own physiology to mold their bodies to the personalities (IE: giving themselves conditions such a shortsightedness and diabetes), sorta a "mind over matter" deal. The Beast is the product of Dennis, Patricia, and Hedwig taking that ability to the extreme.
This is somewhat Truth in Television, since for a while in the 1980s and 1990s, some psychologists alleged that DID patients could change their eye color depending on which personality was out, that a person with diabetes could have a personality that didn't have diabetes (and could produce blood tests to prove it), and that the electric energy the brain generated when alters swapped back and forth could blow light bulbs and cause static on tvs, among other wild assertions. In real life, these claims have been largely debunked...but since this movie takes place in the Unbreakable universe, where superpowers are a documented phenomenon, it could be that these abilities are real. (The more uncharitable interpretation is that Shyamalan did not do the research and presented a lot of outdated and damaging claims as fact.)
Doesn't "Patricia" ever use the bathroom? Wouldn't "she" notice her male body part?
It's pretty clearly established that the personalities are aware of each other and know they share one body.
So is Dr. Fletcher really just a bad psychologist or just too idealistically optimistic for her own good? It seemed like her discussion with Barry amounted mostly to "are you okay," followed by "yes." Additionally, Kevin had 23 different personalities in his head. Was medication ever considered? Apart from that many personalities in head probably not being a healthy thing exactly, and he was undoubtedly suffering from other mental health conditions.
Split personalities apparently doesn't have any recognized medication, it's almost exclusively therapy oriented. Despite the premise of a Warrior Therapist, most therapy is passively asking questions and letting the patient feel comfortable enough to answer it, there is not much else you can do. Dr. Fletcher is established to have some outlier views on the disorder, but she demonstrates an immense amount of respect to all of the personalities, and she is shown to be suspicious of his behavior from the beginning.
The film seems to want to have it both ways in presenting Casey as being better equipped to survive the Horde's kidnapping because she's a hunter who has survived trauma, but also having to go through character development to be able to "pull the trigger" and defend herself. She passes up several opportunities to defend herself in the first two acts. She could have exited the car in a moment if she'd just done it decisively while Dennis was ignoring her. If Casey had joined with the other two girls in fighting the Horde early on, they had a good chance of succeeding, especially if they had made a combined attack on Hedwig (who revealed himself to be physically inferior) or Patricia (after she'd already been staggered by Claire's chair attack). Her hedging only gets the girls separated and delays long enough for the Beast to arrive. Ultimately, it's the soft, "impure" girls who were the ones willing to fight from the beginning, while the hardened, "pure" Casey only rises to their level in the final act, causing the other girls to get killed in the meantime.
I don't understand what's your headscratcher here. You just described the character arc of the main character.
I'm assuming that the Horde's statement at the end, that trauma has made Casey better equipped to survive, is An Aesop. The film doesn't do a good job of establishing that Casey is better equipped to survive. In fact, it shows the opposite, that Casey has to go through a character arc to get to the point where she's able to defend herself. Her arc ends at the starting point of the other two girls, even though the Horde characterizes them as soft and weak. So my headscratcher is that this doesn't make any sense. Either the Horde is just spouting gibberish or the film has a Broken Aesop.
The "soft" girls die because of course they do, physically attacking an abuser who has control over your environment is one of the stupidest things you can do, because you have no idea how they'll react. If they had attacked Hedwig or Patricia, the only thing that would do is guarantee that a stronger personality would take over and kill them — which is exactly what ends up happening. The goal of survival is to win your life, not pride or dignity. Casey only fights back directly when all of her other options are exhausted— but she still would have lost, because oh right, she was fighting a superhuman with a manky shotgun. The "soft, brave" girls die because they think that they're invincible because they're morally right, even though the closest thing they've ever experienced to real violence is an argument in the comments on a Justin Bieber video. They think they're tough, and they're wrong.
I don't think your analysis of the situation is borne out by what we see in the film. Firstly, the two "soft" girls aren't shown to be acting out of pride or anything similar. They simply realize that they're going to be killed unless they do something, and they're correct. They certainly don't think that they're invincible (refusing to initially attack the Horde without 3-1 odds). And we see from the girls' physical confrontations with Hedwig and Patricia that the Horde doesn't instantly swap to a brawnier personality when he needs more strength, so they do have opportunities to fight back and succeed. If the "soft" girls are supposed to have no idea how to survive a crisis, why were they drawing all the correct conclusions? Secondly, like I say in my comments above, Casey's decision to fight back in the end is presented as the right decision, but she had to go through character development to get to that point, whereas by the Horde's logic, she was "hard" from the start, when she was passively making poor decisions (the most obvious being staying in the car to await her abduction). That doesn't jibe.
The Horde don't base their social Darwinism on ideals of physical strength or courage to begin with. The kind of toughness they value is psychological; having the mental fortitude to bear your troubles in silence, or if all else fails, being willing to choose self-harm or even suicide over allowing life to beat you down any further. Casey does obviously want to escape, but from her perspective, she has comparatively little to lose if she doesn't; if she gets out, she goes right back into the clutches of her uncle. This relative passivity in the face of death probably makes her look braver in the Beast's eyes. And it's important to note in any case that Patricia, at least, is extremely strong; being struck with a chair doesn't faze her that much and it's far from impossible that she could've recovered and overpowered them all anyway. So Casey's lack of action could be both pragmatic and, from the Horde's perspective, righteous.