Victory Guided Amnesia: Except for Mike, all of the children completely forget their victory and spend most of the adults' portion of the novel trying to remember. After their second victory, they all begin to forget again, including Mike, who takes his memory loss as a sign that It is truly and permanently dead.
Bill Denbrough (aka "Stuttering Bill"; "Big Bill")
Author Avatar: Became a successful horror author as an adult, and in the movie one of his books is even called The Glowing.
Big Good: Although the Turtle has this role in the novel.
Determinator: Bill won't let anything stop him from getting justice (or revenge) for Georgie's death.
Heroes Want Redheads: Young Bill and Beverly, both redheads, have a mutual crush. Adult Bill also ends up marrying a redheaded actress called Audra.
It's All My Fault: He has a lot of guilt for being the one who came up with the idea of fighting and killing It and thus bringing his friends a lot of misery. He also blames himself for George's death at Its hands, because he was the one who sent George out to play.
Parental Neglect: After George's death, Bill's parents become increasingly distant and ignorant of him.
Roaring Rampage of Revenge: He initially only wants to kill It because it killed his little brother George. He later expands it to avenge all of It's 1957-1958 victims.
Stutter Stop: In key moments in his childhood. He manages to overcome the stutter completely after the first ordeal, but when the memories of it resurfaces, so does the stutter.
Ugly Guy, Hot Wife: Bill and his wife Audra (at least in the movie, where the pony-tail and glasses he wears in the movie make him look ridiculous).
Scars Are Forever: Zig-zagged. Henry cut an H into Ben's belly when they were kids, and a much deeper scar was inflicted on him a little later. The H scar stayed (and became a neat pub story) but the deeper scar faded away after they left Derry. When IT resurfaced and the Losers started being drawn back to Derry, the scar reappeared.
Domestic Abuser: Beverly's husband Tom. In her first scene, she gets the call from Mike, and when Tom tries to stop her from leaving, she decides she's had enough of his shit. They fight and Tom gets the worst of it.
Hello, Nurse!: It is acknowledged in-universe that Beverly is very beautiful. The reason that she isn't part of the popular girl's clique is that she's poor and can't afford nice clothes, and she doesn't act very ladylike (swearing, smoking, playing out in the woods instead of doing stereotypically girly things, and hanging out pretty much exclusively with boys). Also, the richer girls are rather annoyed that they are getting upstaged in terms of looks by someone who's working-class and from the poor side of town.
Heroes Want Redheads: All the male Losers are romantically attracted to Beverly at some point, as is Beverly herself to Bill and they all have sex with her - although Ben becomes a couple with her at the end.
Butt Monkey: He grows up as a fragile, allergic-to-everything Momma's Boy, gets his arm broken by Henry and pals, and marries an overbearing carbon copy of his mother. It's even worse in the movie, where he never stops living with his mom and dies a 40-year-old virgin.
Calling The Old Lady Out: Eddie is pissed when he's in the hospital and his mother sends his friends away. She is actually frightened of him for a moment. Note that this is the only time he ever stands up to his mother.
Mouthy Kid: Leads him to become a nationally-known radio host when he grows up.
Arch-Enemy: To Henry Bowers. Henry hated Mike the most out of all the Losers. He learned it from his father Butch, who absolutely detests Mike's father Will, mainly for the fact that he's black, but also because he's simply a better farmer.
I Choose to Stay: He never left Derry even after the rest of the Losers' Club had moved, and essentially acted as the watchman in case It ever returned. Though he didn't precisely "choose" to stay any more than the others "chose" to leave. They left because their parents moved away, he stayed because his never did. It was less choice than it was fate and possibly the manipulation of the Turtle.
If You Kill Him, You Will Be Just Like Him: During their showdown in the library, Mike is stopped from killing Henry by the realization that if he does so, he would be doing It's work just as surely as Henry is doing It's work by trying to kill him.
Token Minority: The only non-white member of the Losers Club. (Though it seems more likely that the author was merely searching for archetypal characters likely to be outcasts, particularly in small towns—the fat kid, the wimpy kid, the Jewish kid, the rag doll, the hyperactive foureyes, the kid with the speech impediment, and the only black kid in town. Seems to fit.
Informed Judaism: In the book, Stan tells his friends his family are secular Jews, and do things like eat ham and work on Saturdays.
Oath Breaker: He swore the blood oath to return and take down It along with the other Losers, even being the one to cut their palms so they could make it. Instead of returning to honor his vow, he offed himself. It is also interesting to note that Stan was the one who cut each of the Losers' hands to seal the blood oath and eventually killed himself by slitting his wrists.
Skepticism Failure: Stan is the last of The Losers' Club members to recognize IT's existence. There is the implication that his extremely ordered, rational nature is what led him to choose suicide rather than face It again, a monster that defies rationality and natural laws.
IT/Pennywise the Dancing Clown/The Spider
Achilles' Heel: It has to take a physical form in order to directly influence the world, however this means that It can be hurt and banished (though it's not clear whether It can actually be killed in this manner); also, It relies on a weird form of Clap Your Hands If You Believe, where if someone believes that the form It is taking at the moment has a certain weakness, It develops said weakness.
Ax-Crazy: Is cold, cruel, loves to kill, and is utterly beyond human comprehension.
Big Bad: Is the primary antagonist of the novel and movie.
Blob Monster: When IT emerges from a drain to attack Beverly, she notes IT is a shapeless mass with a taffy-like consistency.
Child Eater: IT prefers to munch on children because their imaginations and emotions are more vivid (read: juicy).
Dirty Coward: IT's brazen enough when it's in control of a situation, but begs and pleads as soon as it realises it's vulnerable, and it's last words (apart from a Big "NO!") are frantic attempts to bargain for it's life.
Eldritch Abomination: IT's true form in the Losers' universe is actually a "spider", or at least the closest thing the human mind can comprehend. What IT really is, in the space outside reality where IT resides, is different and much more brain-melting.
Giant Spider: Not IT's true form, but probably as close to it as any human is able to perceive. It's more of an "anchor" that allows It to exist and influence the world.
Gender Bender: IT's favourite form is Pennywise the Dancing Clown, which is implicitly male, but IT is actually female.
Genius Loci: IT practically is Derry. The fact that Derry recovers following the end of the book is a hint that IT is still alive.
Hoist by His Own Petard: IT shares the weaknesses of whatever form it takes. Also, if several people all perceive IT as one form and think of it in that form hard enough, it becomes "mode-locked" and unable to change. The Losers take advantage of this in the house on Neibolt street. They first lock it into werewolf form, then they drive it off by shooting it with a silver bullet from a slingshot.
Load-Bearing Boss: After IT is finally defeated, a flood destroys much of Derry later that year. It's implied that IT had allowed the town to exist in exchange for providing it's victims every 27 years.
Reality Warper: If one believes enough in IT's illusions they become real.
Red Oni, Blue Oni: Pennywise/the Spider and the Turtle. One actively hunts down and eats human children while the other just sits on the edge of forever, seeing it all happen and "helping" the Losers during their first confrontation with IT. The Spider berates it for just sitting there, offering seemingly useless advice. That the Spider's eyes are described as ruby-red while the Turtle's shell is some blueish-greenish color also reinforces the trope. IT suspects/fears an "Other" beyond the Turtle that is also opposing IT, but it's not until The Dark Tower novels that this is confirmed.
Shapeshifter Default Form: Pennywise the Dancing Clown is a Type B, being the form IT uses to get around and interact with people.
Shapeshifter Mode Lock: Happens to IT when the entire Losers' Club perceives it as a giant eyeball. Notably, one of the characters was about to perceive it as something else, but when one of them shouted "It's a giant eye!" it appeared to all of them in that form. Also happens earlier when they perceive IT as a werewolf, and shoot IT with a silver slingshot bullet.
Smug Snake: IT is arrogant and sadistic when in control of a situation, but when the tables are turned it retreats.
Troll: IT likes to flavor the meat of its victims with fear before chowing down on them, but a whole lot of its behavior can't really be explained outside of the sheer joy it takes in tormenting people.
Villainous Breakdown: Has one near the end of the book When Bill and Richie corner it in the sewers beneath Derry.
Even Evil Has Standards: Several of Henry's flunkies, most notably Victor, are horrified by the lengths Henry is willing to go to torture the Losers, such as carving his first initial into Ben's belly or trying to blow up Mike with M-80s and cherry bombs.
Abusive Parents: Henry's father is a violent racist who treats Henry horribly. As nasty a person as Henry is, it's not hard to see where he gets his behavior from when you look at Butch.
Ambiguously Bi: In one chapter, he lets Patrick Hockstetter masturbate him, which gives him an erection that Patrick claims is the biggest he's ever seen. When Patrick offers him oral sex, however, Henry punches him out and derisively tells him he "doesn't go for that queer stuff."
Ax-Crazy: Steadily grows more psychopathic and unstable as the book goes on. This is not lost on the other members of his gang, most of whom begin to shy away from him because of it. The only one who doesn't is Patrick Hocksetter, and that's because he makes Henry look positively well-adjusted. His father, Butch, is not much better.
Eye Scream: In the book, Eddie gouges out his right eye with a broken bottle.
Freudian Excuse: His father is abusive, racist, and not that much more stable.
Greaser Delinquents: Henry's modus operandi. Contemptuous of all authority, sporting a leather jacket (it's pink in the book) and a duck's-ass haircut, and menacing weaker kids with a switchblade. He's also a big fan of Rock & Roll, which is one of the few things he and the Losers agree on, though none of them realize it.
Mean Character, Nice Actor: Jarred Blancard as Young Henry Bowers. According to the DVD commentary he felt really bad for having to use the N word and couldn't be more apologetic to Marlon Taylor (Young Mike) after they finished filming the scene.
Politically Incorrect Villain: He hates Stanley because he's Jewish, he hates Mike because he's black, he hates Eddie because of his asthma, he hates Beverly because she's a girl, he hates Ben because of his weight, he hates Richie because he's a little smartass four-eyes twerp, he hates Bill because he's a stuttering nerd, we can go on forever.
Real Men Wear Pink: He wears a pink motorcycle jacket in the book, and woe betide any kid foolish enough to laugh at it.
Schoolyard Bully All Grown Up: After spending many years in psychiatric hospital, Henry Bowers escapes with IT's help and almost kills Mike Hanlon.
Teeny Weenie: Compared to Victor, as Beverly notices when she watches them light farts from a hiding place.
Sanity Slippage: Throughout the summer of 1958, Henry slowly but surely loses it after suffering numerous defeats at the hands of the Losers. By August of that year, he's completely snapped.
The Sociopath: He's contrasted with his two primary associates in bullying, Victor Criss and Belch Huggins, in that Victor and Belch like picking on the other kids, even beating them up, but they don't want to do any lasting harm. Once he gets angry enough, Henry simply doesn't give a fuck about the consequences.
Would Hit a Girl: Henry is equal-opportunity when it comes to getting back at anyone who pisses him off.
Even Evil Has Standards: Victor is fine with beating up smaller kids for fun, but shocked by some of Henry's actions (such as trying to carve his name on Ben's stomach with a knife).
Gag Penis: Beverly makes note of it when she sees Henry's gang lighting farts from a hiding place. Contrary to the trope name, it is NOT played for laughs.
Heel Face Door Slam: There are some hints in the book that Victor was considering defecting to the Losers, and may have gone as far as warning them about Henry's deteriorating state. If he was considering this, though, IT put a stop to that.
Noble Demon: Not a completely straight example, but he has some shades of it. He has no problem beating other kids up, but, he will never go as far as to do permanent damage to them. While being chased by Henry's gang before the Rock Fight, even Mike acknowledges that out of all of them, at least Victor doesn't want to do him any serious harm.
Off with His Head!: When they meet It (in the form of Frankenstein's monster) in the sewers, the first thing it does is rip Victor's head clean off.
Only Sane Man: Victor is among the first to realize just how far off the deep end Henry is going, and likely the first to do so on the bad side.
Red Oni, Blue Oni: Although never acknowledged as equals, he is the Blue to Henry's Red. This is especially clear in the Rock Fight. Henry got hotheaded, and only got himself hurt even further. Victor kept his cool, and was able to take all the damage Henry's Gang suffered, and return it in kind back to the Losers by himself.
Even Evil Has Standards: Beating up smaller kids for fun is fine by him, but he is pretty disturbed when Henry tries to carve his name on Ben's stomach.
Gasshole: Hence the nickname.
Heroic Sacrifice: Even though he's been a bullying Jerk Ass for most of the book, Belch actually redeems himself when he protects Henry from IT after the monster kills Victor and goes after Henry. Unfortunately for him, he gets half of his face torn off for his trouble.
Fearless Fool: because of his mental state, Patrick has little understanding of the concept of fear. Because of this, IT is not quite sure what form (besides the flying leeches) to take when it attacks Patrick (Patrick notices the thing approaching him is constantly changing shape, as if not sure who or what it wants to be).
Karma Houdini: Discussed and ultimately averted. When his younger brother was born and Patrick lost some of his parent's attention, he smothers the baby with a pillow. His father did become suspicious at one point, but he ultimately decided not to take action against him. No one ever finds out, but Patrick eventually meets a grisly fate courtesy of It.
Karmic Death: Is attacked by It in the form of a swarm of leeches, passes into unconsciousness, and comes to while It is busily devouring him.
Psycho for Hire: Emphasis on "psycho". Even Henry (who is not a bastion of sanity himself) is deeply disturbed by him.
The Sociopath: Even moreso than Henry. He thinks that he's the only real person in the universe (Solipsism), and the only thing that can excite him is killing and torturing other creatures.