Cinder is a particularly unfortunate case in RWBY, even by their shitty villain standards. There's a lot RWBY gets wrong on a fundamental level; their worldbuilding is nonsensical when put to any sort of scrutiny, they don't know how to characterize or write for an ensemble cast, their powerscaling is stupid etc. etc.

having laid all that out, Volume 1-3 Cinder, baring a couple issues around Qrow and Ozpin, isn't subject to any of that. Around this sub we seem to think of Cinder as having always been a bit shit as a villain, but that's not actually true. In the first three volumes she's a victim of particularly shit voice acting and the turbo jank animation that everything suffered from in the early days, but the bones of a good villain are there and what's more they're strong.

The establishing shots for your shadowy big bad are pretty straightforward and Cinder checks them all with flying colors.

1. She's strong
2. She's smart
3. She's making movements

Torchwick is afraid of her and she's got him under her thumb, she successfully manipulated the White Fang for her own ends, infiltrated Beacon, and had she not got played by Salem at the last hurdle (We'll talk about this in a sec) would've fully succeeded in taking Vale out. She's a successful villain, a good villain even, right up till they punt in Volume 4.

And boy do they punt it hard. Cinder turns into a simpering weakling with no thoughts or plans beyond hating Ruby and wanting to kill her. She doesn't even seem to resent Salem, and even in later volumes they only seem to take issue in that Salem has big picture shit she wants And Cinder just wants to kill Ruby and get power for the sake of having it, probably so she can kill Ruby some more.

The first thing to fix about Cinder is her motivations and plans post Volume 3. It's fine for Cinder to be interested in ruby, but she should only have that interest insofar as not wanting to get one tapped by silver eyes a second time or potentially wanting to use them as a means to get Salem's grim influence out of her body. Once she was done picking her teeth up of the floor her thought process should've gone something like this.

1. What the fuck was that?
2. Why didn't Salem warn me about it?
   1. Well because she gets everything she wants this way. I accomplished her goals of collecting the Fall Maiden's power, killing Ozpin and breaking a world power, meanwhile I'm crippled and have grimm in me she can control.
3. Salem played me. I need to beat her and find a way out.

Following this line of thinking the writing fixes itself. 'Salem wants Relics? I want them instead.' 'Salem has an army of grimm so I need more allies.' 'Salem has power over me with the grimm implants so I need them gone.' Setting clear goals for her was just that simple, and they even still align with most of the actions she took in the actual show. She's just less of a sniveling moron and more a real side villain. I don't even think it's fundamentally wrong for her to kind of be flying by the seat of her pants. She's in a pretty tight spot trying to stay under Salem's radar as an active traitor. Gotta take what you can while the taking's good when you don't have a lot of room to work with, but CRWBy didn't bill it to us that way so it falls flat like she's just a run of the mill bitch for its own sake.

Next issue is the backstory. I know she's supposed to be Cinderella, but like come on. If you wanted to keep the analogy in tact you'd be much better served starting from the other end of the story. Her Prince Charming is dead, her fairytale ending destroyed, then along comes her very own fairy demon mommy with the promise of her picturesque life back and the power to protect it this time. More compelling, more sympathetic, gives her something to fight for with some conviction which she desperately needs. Also fixes that thing where she needs to be older than like 20 to sell her lines convincingly. I swear to god why didn't they just start with an older voice actress....? Or don't tell us her backstory at all since we don't really need to know it. Not every villain has to have an interesting past. It would still be an improvement over what they tried to sell us.

Lastly we're gonna get out ahead of the inevitable 'redemption arc' they're gonna try for her at some point. This is actually laughably easy to do. Y'know how in every Sonic came Eggman and Sonic are at each other's throats then a way bigger douchebag shows up and Eggman immediately does a face turn till said douchebag is dealt with? Just do that. Eggman doesn't turn into a good guy, just a guy who still lives on earth and doesn't wanna see it destroyed/enslaved/what have you. Oh would you look at that CRWBY have a perfectly good demon mommy shaped douchebag just sitting right there not contributing to the plot. I wonder what that could be used for? Just can't put my finger on it I swear.

TLDR: I'm annoyed CRWBY managed to punt a perfectly good villain so hard. Rewriting her within their own established story to be more sensible took me like 10 minutes however long it took to type this post. Sheesh.