Karma!

So Courtney is one of the very very very few characters in Total Drama who gets character development that actually sticks. And, granted, a lot of this development isn't positive, but the fact remains that her characterization only changes in accordance with the events she experiences and the lessons she learns from such.

Almost every other Total Drama character who receives a modicum of character growth has that development wiped away for the sake of story convenience.

Leshawna learning to put aside her differences with Heather in Action? Gone by World Tour.

Cody learning to respect Gwen and her relationship with Trent? Gone.

Sierra learning to treat the people around her like, well, people instead of objects of her fixation? Gone.

Duncan's whole deal in Action was that he's more than just a rebellious chud and... Well. I've heard horror stories about All-stars.

Lindsay learning to stand up for herself and take charge in Action, proving she's more than just a dumb blonde? ... To be fair Lindsay wasn't so much reverted than outright flanderized, but you get the point.

Courtney never receives this treatment. Probably because the character she becomes is a lot more compelling as an antagonistic force than the character she started as, but the fact remains that she's allowed to keep those characteristics; The Courtney from Island was bossy and stand-offish, sure, but it wasn't until after her rigged elimination that she became outright aggressive. She changes, and those changes stick.

Her bossiness morphs into obsessive demanding, and almost every time she breaks out a legal threat she's rewarded with what she wants, or at the very least a compromise in her favour. She learns from her comeback in Action that contracts and lawyers and the threat of civil court can and will solve a lot of her problems, and this consequently worms its way into her everyday life to the point she starts writing up contracts and requirement forms for her personal relationships. Unfortunately, she never learned that trying to control your partner through legally binding contracts was absolutely insane. Mostly because Duncan kept getting back together with her before she could ruminate over the idea that she might be the problem.

Speaking of relationships, it's a given that she became a lot more lenient to rule-bending and sabotage after leaning such from Duncan. That was, like, a whole plot point in Island that follows her throughout the series. Doesn't really need to be discussed, but it would've been so easy for the writers to forget this and have Courtney go back to being straight-laced and incapable of comprehending someone cheating/stealing/bending the rules.

She also becomes increasingly more physically competent throughout the series. Island's Courtney couldn't handle the idea of green jell-o whilst World Tour's Courtney was helping Alejandro cheat in the gross-out food challenge with her "iron stomach" (I've also heard something about ice cream in All-stars??). Island's Courtney wasn't particularly athletically gifted in comparison to the rest of the cast; in Action she quadruple back-flips through a room full of lasers, and in World Tour she literally surfs down a waterfall during a volcanic eruption while singing. Does this athleticism come out of nowhere? Not exactly—she boasts about her CIT training a lot, but it isn't until the vengeance-fuelled fire is lit beneath her following her first elimination that she starts showing her capabilities rather than telling. This isn't so much a lesson she learned, but it is a consequence of her surroundings; if Harold hadn't gotten her eliminated unjustly, she prooobably wouldn't have gotten mad enough to unleash her final form.

She also notably learns not to put any stock in interpersonal relationships (outside of Duncan). Courtney always had a pretty tough time making friends in the series, Bridgette was the only girl on the Killer Bass team I recall being her friend rather than her begrudging teammate, which is mostly due to Courtney's strong personality. In Haute Camp-ture, Lindsay even goes so far as to point out that nobody likes Courtney, which she seems to take to heart. In Action, she isolates herself from the rest of the contestants and focuses solely on winning. In fact, the only time I remember her being friendly to someone else was when she was trying to form an alliance with Beth, and even that was just a means to an end rather than an actual relationship. Then in World Tour, it takes the better part of the season for her to let down her walls around Gwen, and that was swiftly thrown back in her face; she learned from Island (specifically Haute Caamp-ture) that being friendly and helpful will get her nowhere because people will dislike her regardless—look back to the literal first episode of the series, where Courtney is the first and only person who runs to assist an injured Izzy at the docks. The Courtney of later seasons would see an injured Izzy and think "good, that's one less person I need to compete against". This is learned callousness. And this persists into every season she stars in, because that lesson only ever gets reaffirmed.

courtney and duncan's relationship is one that's destined to fail from the very beginning. they're both stubborn, hotheaded, and prideful people who are very eager to put the competition and money ahead of their relationships with people, each in their own ways, and butt heads with each other often. all of this is specifically exacerbated and exploited by total drama's format; neither of them know how to communicate with each other, but it benefits the show for them to be at each others' throats. they aren't given an opportunity to settle down and look at things rationally before the relationship reaches a point of no return, specifically fueled by courtney's fiery and unyielding personality from action onwards and duncan's refusal to display vulnerability and softness lest he taint his tough image. they're at an impasse.

they both recognize that there's something to remedy in their relationship and that they want to be with each other around princess pride in action, but it's not until top dog that they sit down to talk about it, and out comes courtney's dreaded contract. as op points out, this is the only way courtney's learned that gets her her favour: legal rigorousness. duncan doesn't protest this to her because he's interested in their relationship, but he also is clearly unhappy with it as he speaks to owen and in the confessional about it. by the end of the episode, he's decided he's done playing with this, but instead of telling courtney about it at all, he votes her off the show.

their back and forth, on again and off again relationship persists from the finale onwards through celebrity manhunt, until they're in an ambiguous state at the beginning of world tour. duncan likes the relationship in this state, as it's where it is in action when courtney confronts him in million bucks, b.c., where he states that his relationships are "none of her business" because it gives him access to courtney's attention when he wants it, but also plausible deniability when he behaves less than faithfully. or, eventually, outright cheats.

the difference for courtney that she experiences for the first time is to be in the competition without duncan after he quits. it's the first time since they get together in island that she has no time to fixate on him, either competing with or against him on the show, or by watching him from the audience. she has to focus on the competition.

and world tour wants to tell you that courtney, without the intensity of their relationship or their competition, thinks really fondly of duncan. she turns down alejandro's advances immediately in walk like an egyptian ("nice try, but i'm with duncan") and doesn't even join in when alejandro tries to undermine duncan by telling her she "deserves better than a quitter." she's excited at the thought of seeing him in newfoundland, to the point where the girl who sues her way back into a competition (not financial compensation!) and values winning over her relationships abandons a challenge just at the chance to see him. sierra goads courtney into admitting that she misses him, a lot, she says softly and sincerely. we later see her admit wrongdoing in the confessional later in the episode, where she starts to display vulnerability and even guilt over potentially being at fault for duncan abandoning her and quitting the game. admitting fault is something courtney never does.

the separation she gets from duncan for the first time has given her time to reflect, to realize she does value her relationship with him, and likes him for who he is, despite how prideful they both are and the issues he might have in her eyes. she does jump to scolding him as soon as he's back in the game in i see london, but immediately drops it to hug him and admit that she missed him. she's hard-headed and critical, but she's starting to let her guard down and let herself care about him free of the image she has of the "right" relationship in her head.

in the next episode, greece's pieces, she gushes about him to gwen. and although she brings out another list, we go down from a 32 page contract to a list of 47 items. still...misguided...but she's learning! and she even tells gwen later that she's starting to not care about these things either (when she says she originally wants his mohawk to change but actually loves it, and how "love does funny things")

it's a gradual shift, but i think she's supposed to be growing on her own here despite all her pride. the development isn't complete, but if she were given more time it might have bloomed into a much healthier, more honest relationship between her and duncan.

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