"I won't spoil the conclusion if you're eager to see this through."
This article contains MAJOR SPOILERS relating to
some of the most pivotal parts of Worm. Although all articles may contain spoilers, this article will reveal major plot twists and should almost certainly be avoided unless you have finished the story.
Parahumans are humans who have undergone a traumatic experience (known as a "trigger event") and awakened superpowers. The slang/colloquial term for parahumans is "cape", which is typically (but not always) used to refer to people who wear costumes, but may refer to Rogues.
Fundamentally Parahumans are gestalt or symbiotic organisms. a combination of human and a symbiotic, and in some cases parasitic, Agent.
A Parahuman functions like a regular human but unlike regular humans, they have a Corona Pollentia. The Corona Pollentia is the portion of a parahuman's brain that adapts to and allows the parahuman to control their abilities. The Corona Pollentia seems to awaken during a parahuman's "trigger event" where it forms a connection with the Parahumans agent is formed. It also seems that parahumans can pass on a similar power to their children without them having to suffer severe mental trauma. It is important to note that while children of parahumans are more likely to gain powers, it is pointed out that it is "likely more to do with exposure to parahumans at formative ages than genetics."
Trigger events can radically alter a parahumans biology as seen with Brutes and Changers, as well as having other effects like reducing or even removing the need to sleep.
Parahumans live in a state of Irony. Their abilities driving them into the same situations that gave them their powers in the first place.
Parahumans who were isolated and not regularly challenged had a mild tendency toward increased mental abnormality. Which makes getting multiple members working together to be a large issue. Further they were know to be more emotionally volatile, their peaks and valleys more exaggerated. This is partially due to the influence of their Agent.
Parahumans are independent people with only their agents influencing them, It innfluences their cognition on a instinctual covert level  Overt control of a Parahuman by their agent is rare.
It is implied that there are a great many capes in the world, but that they still form only a small percentage of the population. Brockton Bay has roughly seventy parahumans that have been introduced and described in the course of the story, implying that capes comprise less than one percent of the population.
It is also mentioned that there is about 1 parahuman for every 8000 in urban areas.
The term Parahuman is one of several descriptions used for superheroes in media. One notable use of Parahuman is in the Miricleman Reboot authored by Alan Moore and later expanded upon by Neil Gaiman. This series also dealt with a realistic portrayal of how Superheroes would effect the World, Politically, culturally, economically and more.
↑“Yeah. I feel like a lot of people can just jump into it, and I don’t. In everything I’ve ever done, I’ve had a bit of a learning curve. School, work, I’m… am I shooting myself in the foot if I say that I’m slow? Not stupid slow, but… slow on the uptake, slow to adapt.”
“You’re not shooting yourself in the foot.”
“It’s a little ironic, though,” she says. “Being slow when I’m a mover.”
“It’s a lot ironic, if it’s true and you’re not being unnecessarily hard on yourself. I think irony is something of a trend, in parahumans.”
“I think it has to do with the trigger events. One event, to serve as a starting point for a new life? From your weakest moment to having raw power? Irony’s inherent in that. There’s usually an inverse link between background and the resulting power. If you talk with your teammates, I think they’d corroborate on some front.”
↑"Capes have a way of getting weird if they’re too isolated.” “Ah, you read Weiss Four,” Dr. Rowland comments "Yeah.” “Explain?” Harrigan asks, leaning forward. “If a parahuman isn’t challenged, they tend to veer a little off course, mentally speaking,” Rowland says. “It’s not a strong pattern, but it’s a pattern. The paranoid get more paranoid, the aggressive get more aggressive.” “That’s a fact with almost any criminal,” Ingram chimes in. “True,” Rowland says. “That’s one of the big arguments against the idea.” - PRT Questthread III page 18
↑Wildbow:Parahumans are naturally inclined toward conflict, because that's why they have powers in the first place - the entities want to test the powers. A great many parahumans are great balls of neuroses and they've got passengers in their heads that may be nudging them a little one way or another, powers that aren't necessarily controlled or easy to manage, or unfortunate implications. - Conversation on Spacebattles by Wildbow
↑““And while I don’t like the way the idea is often interpreted or the conclusions it’s taken to, there’s the notion of volatility, and the exponentially increasing chance of trouble as the groups of capes grow larger. With parahumans, things are often exaggerated, both in weak points or the hot button issues they have, or their inclination to push certain buttons. The more you put in one place, the higher the chance of the wrong button being pushed. That was another concern of mine.” - Excerpt from Flare 2.5
↑Depends on the shard. Bonesaw elaborates on the idea by noting 'breadth and depth' in her interlude. If the shard gets you while you're young, it can shape your personality across the board, on a deeper level. The more conflict you're involved in, the more toeholds it gets to rewrite your consciousness and your subconscious. To alter your thinking, it needs to do it as a part of the trigger event, or as part of the brain's development.
In the extreme cases, the shard can leave you with an impulse (Must fight when a fight presents itself), help set up an obsession ("Wall myself in!"), steer a neurosis in one particular direction (specific hallucinations rather than random ones, of you hurting people, pushing someone down the stairs, etc), create a link between A and B (Being around fire makes subject lose empathy and inhibitions. With lower empathy and inhibitions, subject uses power to make more fire.), or steer a personality trait to an extreme (Must be on top, I answer to no one!), or they just overwrite stuff (Can't understand humans, only dogs).
In the lesser cases, it can be a nudge, hard to distinguish from one's own psychology. You might be on the fence about something, trying to make a call, and the passenger pushes you one way over the other, based on your own feelings of doubt or fear. It might tap into emotions, and dampen X emotion while promoting Y, just dampen them across the board, or take the joy out of day to day living while adding excitement to the cape life. A vague sort of depression that only goes away when one's out and fighting. Sometimes, as mentioned before, it's set up as a trap, a flood of emotion or a set of mental switches that get thrown when a prerequisite is met - such as a cape just steering clear of all confrontations, except the shard set it up so they can't, and they have a sort of limit break/command cutting in that mandates them to fight in one way or another. Or it plays off a limit or a berserk button that already exists - Damsel can't spend too long being anything less than top dog or she gets restless, and if she goes too long despite that, then she has to act, she's acting without thinking about it. This takes time and effort for the passenger, and a host that doesn't demand that time and effort (by circumstance or intent) is going to develop a better connection with the power. This in turn is a reward of sorts. If Damsel did kill the local capes and assume control over the area, fighting off all comers, she'd find her facility and control with her power just ramped up like crazy.
It varies from cape to cape and shard to shard, and it varies depending on the host, the host's background and the host's personality.
Beyond that, other influences include the passenger playing fast and loose with the power itself, as it controls the metadata, which may be more visible if the subject breaks from their norm in terms of consciousness (gets a concussion, tranquilized), working off base instincts and impulses like 'stay camouflaged' (be a little more creepy and unsettling), intimidate/dominate (passenger works behind the scenes to make you look a little more dangerous as you mutate/grow/surround yourself in the aura of your power), etc, etc. In more pronounced cases, the power is just plain controlled by the passenger, not the host, and the passenger makes the seemingly random or uncontrolled aspects generate more conflict... pushing a power to kill rather than leave someone alive, or a thinker power turns up a vision of something the subject didn't want to see.
On the macro level, too, don't discount the fact that some shards (particularly powerful ones that warranted attention) are just sent to specific people, with the idea that it's a combination that's going to promote more conflict just by the sheer dynamic of it (Powerful person with a destructive power, a desperate person with a power with negative implications). - Comment by Wildbow on Spacebattles
↑ Shards only very rarely 'make' people do anything. They pick their hosts with care, those people who are going to be inclined to use powers more or throw themselves into a given type of situation, they may nudge, or encourage more subtly, reinforcing behaviors they want with more power, more focus and utility in the power, or in damping down any drawbacks. In some cases, they may ebb and flow in terms of effectiveness, and in cases like Canary's, may ebb more for a long time, getting her to let her guard down, before a 'kill all the Japanese' chance comes up. - [Spoilers All Did Canary's shard...?] Wildbow, Reddit.com, 2016
↑The broadcaster had finished speaking just a moment before the craft had launched, oblivious to the blaring noise that had been intended to drown him out. What I don’t understand, is why a blank slate like you would default to doing good deeds, rescuing cats from trees. Why not turn to that violence, as our ancestors did? It drove them, just like it drives the basest and most monstrous of our kind.
Had he known he had a listening ear? Had it merely been a struggle to continue doing what he’d instinctively done for decades?
↑Insurmountable. Too much work for one woman to handle. She delegated where she could, but too much of the responsibility was hers and hers alone. The humans outnumbered parahumans by eight-thousand to one, give or take, in urban areas. Outside of the more densely populated areas, it dropped to a more manageable one to twenty-six-thousand ratio. But here in Brockton Bay, many had evacuated. Few places in the world, if any, sported the imbalanced proportion that Brockton Bay now featured. What was it now? One parahuman to every two thousand people? One parahuman to every five hundred people? Each parahuman represented their respective interests. She represented everyone else’s. The people without powers. - Excerpt from Interlude 13
↑It's worth stating that one of the underling ideas driving the formation of groups and ideological factions in the Wormverse is the notion that some people get a voice where they otherwise wouldn't. Give an immense amount of power to a (relatively) random section of the population and you'll see certain shifts in the overlying sentiments.
All the more so when you think that a parahuman with aspirations might latch on to an idea, concept, or group to get reputation, resources, and contacts. - Comment by Wildbow on Sufficient Velocity