In this AU, The Princesses DIDN’T hand off their powers to Twilight, and still remain the primary Rulers of Equestria during the course of the story
 
 
Cozy Glow was born premature to a unicorn mare named Sugar Sweet* and a Pegasus stallion named Bluebell.* Sadly, Sugar died soon after the delivery due to complications while giving birth. Bluebell, scared and confused, wasn’t ready to care for the baby alone, so he passed her off to a local orphanage with the condition that they call her by the name her mother had chosen just before her death.
 
Cozy, though much smaller and younger than most of the other foals at the orphanage, was naturally gifted at commanding and persuading ponies to follow her lead. Think of it like the story of Little Orphan Annie, though unlike Annie, Cozy’s life didn’t magically improve at the drop of a hat. By age ten, most of her friends had either already been adopted or “graduated” and were starting a life of their own. After a while, Cozy’s sadness turned to anger and resentment: “Why am I always the last one picked? Is it because I’m too small? Too weak? To Tartarus with ‘em! I’ll show everypony how powerful of a leader I can be, and then they’ll regret ever turning their backs on me.”
 
Some of the older ponies in her dorm had taught her how to bribe and negotiate. They said it was the only way to truly succeed in this world, and she believed them wholeheartedly after years of “We’ll pass” and “Sorry, I just don’t think she’s the right fit for our family.” Using the knowledge she’d obtained from those higher-ups, Cozy Glow began to observe the surrounding employees of the facility. Eventually she landed on a social worker who’d been having an affair with one of his coworkers (she’d obtained this information through constant eavesdropping of his conversations with said mare). Cornering him in his office one day, Cozy blackmailed him into letting her escape under the clause that she’d reveal to everyone the juicy details of what exactly he’d been doing in the broom closet with Ploomette every Friday afternoon…
 
This was story for how she got her Cutie Mark in manipulation - and it only got worse from there.
 
After her escape from the orphanage, Cozy faked an ID for Princess Twilight Sparkle’s School of Friendship, where she noticed three young fillies were having trouble applying for the academy themselves… and that’s precisely how she was able to climb the ranks from average student to assistant to Princess Twilight herself.
 
And the rest is history - or is it?
 
 
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At age 19, after Flurry Heart fled the Crystal Empire (she couldn’t take the constant scrutiny and impossible standards she was held to as the “miracle princess of the Frozen North” blah blah blah) she made sure to stop by the Castle Labyrinth before traveling south to Ponyville. Flurry had always felt guilty for Cozy Glow’s sentencing to an eternity imprisoned in stone, cursed to forever be aware of her surroundings but having no way of communicating with those around her; a sort of purgatory or limbo, if you will. And, of course, Flurry was all-too familiar with the concept of curses, so her empathy for the wayward filly only grew.
 
So she freed her, creating a decoy statue in place of where Cozy once stood.
 
Having been frozen in time for all those years, Cozy Glow still had the body of a 12-year-old but the mental maturity of somepony twice her age. Her first reaction to this newfound freedom was to aggressively berate Flurry for her decision. “What is the meaning of this? What do you want from me? I need answers, you pink freak!”
 
Flurry responded by saying that she was going to try her hoof at reforming her herself, and that she now had a second chance at life, something she had no reason to decline or take for granted. Still on-edge, Cozy Glow wanted to know what was in it for her, and what the two of them would get out of this arrangement. Flurry only replied with, “You’ll see,” before leading her to the train station under a spell meant to disguise both of their natural appearances to everypony but the caster themself.
 
A reservation had already been made to stay at Sugarcube Corner for the time-being, and Cozy Glow was subsequently introduced as a young relative of hers with no place to go for the winter. Pumpkin Cake gladly accepted the newcomer along with her old friend Flurryheart, and it wasn’t until after the two had gotten together that it was revealed who “Snuggle Shine” (very creative, Flurry) really was. Though she was initially frightened at the fact that she’d been housemates with a convicted felon for a year, Pumpkin soon realized that Cozy had made quite a bit of progress during her apprenticeship, and allowed her to continue living with them.
 
Cozy Glow is pictured here at her current age - 18 - and while she still retains her sarcasm, anger issues, and occasional bargaining tactics, Flurry raised her to become almost an entirely different pony. They still have quite a bit of work to do in altering a mindset that had been engrained in her since childhood (a “kill or be killed” mentality that helped her to escape in the first place) but Cozy is far from the filly she once was.