My experience with this character below.
It broke my heart when she betrayed me. I rescued her and Gandhi when Egypt and Britain had them both hammered back into their final couple cities. I sent workers into her territory to build up her infrastructure, liberate her cities and gifted them back, gifted her gold and resources as well. I did this for her and Gandhi both.
For most of the game, near start until after winning they were both my closest and only real allies. Meanwhile Caesar and I were at the height of a cold war. We directly bordered each other and had a massive military build up just a space or two away from each other. Both of us had Nuclear weapons armed and ready, fleets standing off with each other.
The Iroquois decided to attack Gandhi and so a war in the African Continent started. Dozens of turns later Caesar finally backed down and I was able to begin shipping my massive sea fleet towards Africa to help my ground forces fight the Iroquois. Catherine who had been all smiles and friendly suddenly declared war on me and bluntly said something about not seeing any other, better way to show her deceptive ways and even though the odds were against her it would be war.
It's kind of surreal how the AI thinks in this game. It's a coincidence I'm sure, but it seems like the moment she realized my fleet and nuclear weapons were no longer tied up with Caesar's cold war, she knew the only chance she had of winning was to declare war there and then and try to take some ground before my fleet could cross the ocean.
Next thing I knew she had artillery and infantry pouring across her border and through mine (I had conquered the area of India and she was based in southeast Asia.) That little incursion didn't last long though, as I had mechs and a nuclear submarine on standby in the area in case they were needed in India or in case the Ottomans to the north had decided to invade.
My bots made short work of her infantry and artillery, it was really like one of those futile soviet human wave tactics... Then I went ahead and nuked both of the cities I had just gifted her less than ten turns before.
The conflict continued and several turns later, all of the Russian territory (south east Asia, again) was little more than fires, fallout, and towers of smoke. She had lost every city but Moscow which was on a peninsula. My mechs had the city surrounded and she had nothing left but said city. My fleet had long since arrived with battleships, nuclear submarines, and aircraft carriers which shelled it for several turns while I hunted down the last of her naval and worker units.
Finally the city was attacked and captured. The last thing my "friend" Catherine ever saw were German mechs stomping through her burning countryside of mines and farms, land she was so certain would cease to be the boundaries of her empire, rather the heart of it some time later.
... Then I get the little diplomacy cut-scene and she's flirting about being my captive as she always does when defeated.
Goddamn it Catherine you didn't have to start fucking thermonuclear world war if all you wanted was to pack it all in and move to Germany. Goes to show you, "bros before hoes." Gandhi had my back during the whole conflict and still does even after. With his help we've driven the Iroquois and their city state allies down into a tiny spit of land at the tip of South Africa where they can be more easily observed and contained.