Like all potential candidates for the Games in One, Cashmere began training at the Academy at a young age. By the time she was sixteen, it was clear that she and Gloss were top contenders to be allowed to represent One. With so many people in One vying for the right to be a tribute, the only way to secure the right to volunteer is by defeating the other potentials in a single-elimination tournament. Traditionally, One only sends 18 year old to the Games. When they were seventeen, Gloss challenged and defeated the nominated male tribute, earning the right to go to the Games early.
Gloss wins the 63rd Games and comes home different in a way that's hard for her to fully understand. She becomes immediately fearful that she will lose him to his new life as a Victor, and realizes that her chance to follow him into that life disappears a little more each day. She moves to the Victor's Village with him and never mentions her plan to volunteer. He doesn't know until she's on stage at the 64th reaping that she'll be going to the Arena.
Her arena is a deadly swamp. The entire thing is messy and horrific, but the spots they replay over and over again are Cashmere drowning the boy from Six in the murky waters, and her screaming like a banshee while her district partner is torn to shreds by a crocodile - then her beating the life out of the beast in retaliation. As her mentor Gloss fights nearly as hard as she does and there are more than a few people that say he won her Games for her.
She wins and other than becoming one half of the most famous siblings in Panem, life doesn't seem to change much. For a few months Cashmere thinks she has everything she's ever wanted.
It's not until her Victory Tour (a whirlwind where she gets drunk for the first time, and indulges in the rich food at every stop - after all, she's earned it) that she learns the price of winning. One the last night, her escort shuffles her off to a private meeting with Snow. He lays out his expectations in no uncertain terms and threatens her parents and Gloss. Because she doesn't want to made into an example like Haymitch, she agrees. He sells her for the first time that night. She feels cheated and betrayed but doesn't tell anyone what's happened. Two years pass before she finds out that Gloss has made the same deal with Snow.
She drinks heavily on nights she's "working" and takes the drugs her clients offer her. It's not long before she develops a way to survive: disassociating herself from her body and what's happening around her. After that, she forgets their faces and everything but the worst nights fade from her memory.
Not many people would describe Cashmere as friendly. If she's decided that someone can't do her any favors, they're likely to bear the brunt of her mean streak (which is pretty wide). She mocks most of the other victors to their face, especially when drunk. One-on-one she can be much kinder. She's jealous of those who thinks are allowed to have the quiet lives of luxury that the victors supposedly have earned.
She mentors more often than she would like, and when she's not mentoring they request her presence in the Capitol anyway. She tries hard to help her tributes win, but often hopes they don't. She knows that the beautiful ones will end up like her and it's a relief to watch them die.
Cashmere doesn't volunteer for the 75th Games, but she's not surprised when her name is called. From the moment Snow announces the Quell, she knows she's going back in. She trains hard but she never intends to win - the Capitol won't let themselves be tricked into having two victors again and there's no point in living without Gloss. She promises him, in together, out together and when Katniss' arrow hits it's mark she doesn't think twice before throwing herself in the path of Johanna's axe.
TL;DR + misc
♐ last name: Ritchson
♐ her parents are vintners
♐ won her games at 18
♐ mentored by her twin brother, Gloss
♐ they are rarely seen out of each other's company for more than a couple hours at a time
♐ would do literally anything to protect her brother
♐ which is why she lets Snow prostitute her (and why he lets Snow do the same to him)
♐ Cashmere and Gloss often sleep in the same bed
♐ never gives a straight answer about the nature of her sibling relationship
♐ enjoys the company of most of the victors even if she's a dick to them
♐ likes to play pranks
♐ broke up with her long term boyfriend Blaze after a threat from Snow
♐ her victor talent is singing, same as Gloss'. deeply embarrassing duos!
♐ she has two coping methods: disassociation and substance abuse