Basic Trope: An ordinary girl is approached by a Mentor Mascot, and given a Transformation Trinket which transforms her into a pretty-clothed superhero who uses her powers to fight evil.
Straight: Junko Watanabe is an Ordinary High-School Student, until she is approached by a Cabbitsel (cat/rabbit/weasel hybrid) who gives her a magic ring which she uses to transform into Pretty Ronin Trope-Tan so that she may fight Queen Eviloverlordia and her horde of Monster of the Week minions.
Exaggerated: Junko as Pretty Ronin Trope-Tan goes into an all out battle to defeat the bosses that are evil Eldritch Abominations, malign gods and goddesses, and even Death himself.
Downplayed: Junko's magic ring doesn't transform her into Pretty Ronin Trope-tan so much as lend some of the Pretty Ronin's powers. She can help in the fight Queen Eviloverlordia and her horde, but not to the extent the pre-established Pretty Ronins can and do, and in any case eschews the elegant clothing costumes because she finds it embarrassing. Overall, she is more akin to a reservist compared to the other Pretty Ronins.
Justified: Junko is a descendant of the one who originally defeated Eviloverlordia, so naturally only someone of her family line can defeat the Big Bad this time around.
Instead of The Hero, Junko becomes a Dark Magical Girl that fights an established ensemble of the actually heroic characters.
Pretty Ronin Trope-tan is a seasoned soldier from a Magical Land who acquires a magic ring that lets her temporarily seal away her magic powers, so that she can disguise herself as an ordinary girl named Junko Watanabe and help the other students with their extensive (but mundane) problems.
When not kicking ass, Junko is actually overweight, nerdy, male, and American.
The Pretty Ronin costumes and "magical items" are just pink sparkly versions of actual military uniforms and equipment, as the monsters are not Immune to Bullets.
Queen Overlordia and her Dark Empire are defeated by the end of the first episode, and Trope-tan has to find other uses for her Magical Girl Warrior abilities.
Zig Zagged: Junko's abilities, competence and chosen heroine status in general are brought into question many times due to the local Redshirt Army being actually better suitable for tackling the Monster of the Week.
Averted: Queen Overlordia and her ensemble of sparkly glam villains are quite surprised to see the prophesized heroine of justice Junko face them in a gritty Diesel PunkHumongous Mecha instead.
Enforced: The writers reboot a "standard" Magical Girl story into this trope, in order to attract a Periphery Demographic. Junko's star-tipped wand turns out to have a secondary mode where it unfolds into a Wave Motion Gun, and the situations she faces increasingly require its use.
Lampshaded: "Isn't that girl a bit too badass to be that magical?"
Invoked: Upon obtaining an array of remarkably effective, lethal and non-flashy powers, Junko tries to customize them to suit anime-inspired ideals due to being Wrong Genre Savvy. Hilarity Ensues.
Exploited: Trope-tan milks her newfound status for all it's worth to become a national celebrity and a pop figure icon who is being paid humongous sums for product endorsements and has her own anime series with merchandise up the frilly pantaloons.
No matter what she's called, Trope-Tan is little more than a Child Soldier; the battles she fights are extremely dangerous (maybe even traumatizing); she can't tell if her mascot mentor is telling the truth; the villains seem more like sympathetic Well Intentioned Extremists than monsters greedy for power; she can't tell if her allies are actually her friends; and she's so cut off from the ordinary world by her constant battles that she's slowly becoming more and more unstable.
Trope-Tan manages to overcome all of the above and triumph regardless.
Junko is a clinically depressed girl abused by her parents and/or classmates. The Mentor Mascot finds her and told her she could have a more meaningful life as Pretty Ronin Trope-Tan. She agrees, and on the next day, she finds a new home, with the other Pretty Ronins, whom scare off the bullies, and with whom she starts to fights opponents pretty much on equal terms, rather than the one-sided fights she got into. They form a bond that cannot be broken. The villains might be more misguided than evil, but their plans would still doom the Earth, so they need to be stopped. Fortunately, Pretty Ronins are skilled at non-lethal combat, so the opponents don't even need to be killed. And even better: the Mentor Mascot (or one of the other Pretty Ronins) is a therapist!