Alpha Bitch: Arisu Sato. She, along with her henchgirls, viciously bullies Yayoi, is wealthy, looks to be the type of girl who everyone wants to have, is class president, and is an outright Jerk Ass underneath.
And I Must Scream: Anyone trapped in Yayoi's sketchbook is not going to be free any time soon. The only real way out is for her to kill you.
The Cameo: Homura and Madoka at the end. It is the scene where Homura (in her Moemura form) first found out about the Puellae Magi.
Curbstomp Battle: Yayoi/Sunder vs. anyone; she wins hands down, even the Precures she was originally destined to fight with. Mami vs. Yayoi/Sunder ends with the latter's Mooks being decimated in seconds by the former's Unlimited Musket Works followed by Yayoi getting trapped in Mami's ribbons.
Dark Fic: Normally, Smile Precure is a very sugary sweet anime (considered to be a sugarized version of Yes! Pretty Cure 5). Given the amount of suffering Yayoi is forced to endure followed by her Face-Heel Turn and the deaths/imprisonment of so many characters, this was definitely a darker take on the show.
Dark Magical Girl: Yayoi becomes this after her Face-Heel Turn. She had a tormented life before the contract and it only got worse as she discovered her powers and what they couldn't do. Finally, she snapped and and her sense of justice vanished only to be replaced with a cold outlook on life. Her past feelings continue to haunt her, though.
Face-Heel Turn: Yayoi, by the end. She has all but snapped from everything she had done by then and was now actively destroying everything in her path, including her former friends who she was supposed to fight with as a Precure.
Friendless Background: Arisu is described as being the type who would have lots of friends. In reality, her cruel demeanor prevents her from ever having any. Indeed, she is described as not having friends, but subordinates.
From a Certain Point of View: Once again, this is Kyubey's style of explaining things he doesn't want Yayoi to find out so soon. He he uses it once when Yayoi asked him about the Witch she just fought and Wolfrun's relation to it. Wolfrun explained that he was trying to drain the sempai of her Bad End energy and was himself baffled by the appearance of the Barrier. His actions would cause a Puella Magi to fall into despair which causes their Soul Gems to darken quickly and turn them into Witches. Thus, Kyubey explains that he, in a way, summoned the Witch. And Yayoi was none the wiser.
From Bad to Worse: First, Arisu's bullying reaches a peak and Yayoi contracts with Kyubey to get the power to take revenge. Then she discovers the limits of her powers and accidentally kills Arisu. Next, her attempts at hero work only serve to drive her increasingly insane as she traps more and more people in her sketchbook. Finally, she has a massive My God, What Have I Done? when she realizes, from Mami, that she had become the very thing she hated: A bully and a villain. This all leads to her vanishing and eventually turning into the Witch Izabel.
Genre Savvy: Yayoi knows all about the superhero genre. To wit, she adopts an alias (Sunder) while transformed, keeps track of emergency lines via radio, made maps of the city to appear where there's trouble, and attacks criminals. Overlaps with Wrong Genre Savvy by the end.
Ghost Town: Nanairogaoka becomes this, thanks to Yayoi/Sunder trapping most of the population in her sketchbook.
Heel Face Door Slam: Yayoi/Sunder felt a heavy amount of remorse for her actions toward the end. Mami offered her one last chance of redemption, but Yayoi's despair reached a peak. She vanished from the area, but it is implied she had just fled and turned into a Witch later...
Implacable Girl: Cure March described Yayoi/Sunder as being nearly impossible to hurt and that only Cure Happy was capable of at least damaging her. Otherwise, they were on the run with nowhere to hide, and with an insane girl, who was supposed to be their friend, wanting them dead.
Jerk Ass: Arisu, in spades. To wit, she and her posse forced Yayoi to watch as they destroy one of her sketches while giving her "The Reason You Suck" Speech after toying around and overall tormenting her while she's begging for them to stop. And this is just the first scene.
Kick the Dog: Arisu does this every time she and her posse bullies Yayoi. Yayoi eventually traps her own concerned mother inside her sketchbook out of paranoia.
Kick the Son of a Bitch: It was mentioned in passing that one of her victims was a harlequin who worked alongside Wolfrun for the Bad End Kingdom. He is clearly Joker. Arisu doesn't count since she was killed before Yayoi hadn't started turning evil by then.
Kill 'em All: Yayoi/Sunder eventually traps everyone but Miyuki, Mami, and Kyubey inside her sketchbook. When her Soul Gem darkens and she seemingly explodes in a burst of lightning, all that was left of her was her pen and sketchbook. It was mentioned earlier that the book only keeps her victims alive as long as Yayoi is holding it...
Mad Artist: Yayoi becomes this as she tries desperately to sketch her victims back into existence. All that did was make Mooks for her. Her Witch form, Izabel, is described as such to the point where the trope name itself is used.
Mooks: The Familiars reprise this role for the Witches. Yayoi discovers that she can make these in the form of Sketches by drawing them in her sketchbook. These were the failed results of her attempts to draw her victims back to life and are described as mindless copies.
Mugging the Monster: Once Yayoi makes her contract, she makes sure the next time Arisu and her flunkies bully her is their last... Literally.
My God, What Have I Done?: When she realizes she killed Arisu in cold blood. She gets this several more times as she slowly slips into evil.
Nothing Is Scarier: It is unknown what happened and how Akane was killed, but the implications are still there and the horror remains.
Off with Her Head!: This is how Yayoi kills Arisu. This was unintended because Yayoi was only trying to erase Arisu's mouth to shut her up.
Sanity Slippage: Yayoi, again. It becomes evident as she becomes increasingly paranoid about Arisu's father's attempts to find his daughter. This leads her to imprison every cop who comes to her house and eventually her own concerned mother.
The So-Called Coward: What Yayoi really wanted. She wanted to be respected for who she was. Being called a crybaby eventually becomes a Berserk Button for her, as Cure March found out when she was trying to talk Yayoi into becoming good again.
Shown Their Work: Most people pluralize "Puella Magi" as "Puella Magis". "Puella Magi" is Latin, so English pluralization doesn't really apply. In this fic, it's pluralized as "Puellae Magi", which is correct.
Shrinking Violet: Yayoi. Her shyness and inability to stand up to Arisu's bullying is what drives her to contract with Kyubey.
Start of Darkness: It officially began when she accidentally killed Arisu. It only got worse from there as her sanity slipped and her paranoia grew.
Uncanny Valley: Invoked In-Universe. The Sketches Yayoi creates were described as looking human enough, but their jerking motions and other odd movements made them utterly horrifying and disturbing to view, let alone fight. Miyuki described Kyubey as being unnerving when she met him for the first time.