Eris and the other catgirls we see aren't really Catians. They are advanced biological androids, and they don't know it.
So who are the true Catians? Well, they really do look like cats. They've been on Earth for hundreds, maybe thousands of years, waiting for humanity to mature enough, culturally and technologically, to make contact possible.
Aura (Kio's cat) is one of them and is currently in charge of the mission. But there are quite a few others around.
Some of them are inside the assistaroids. The assistaroids are like tiny mechs, but each has a space-time bubble inside which is rather large and includes full living quarters for the operator. And if the assistaroid shell gets destroyed, the operator remains safe in the bubble.
The reason the true Catians created Eris and the other catgirls is because they came to the conclusion that humans would never be able to accept talking, super-intelligent cats. So they began with human genetic information and modified it slightly to create Eris and the others.
They were created quite a long time ago, and have been kept waiting until the time was right. Some of the early experimentation took place on Earth, and Ichika is one of the early prototypes. She managed to escape, and hid among the humans. She knows the truth, but she's keeping it secret out of fear of the true Catians. They have accepted a truce with her because she's left documentation of the truth elsewhere with instructions for it to be released if something happens to her.
This is why it was so easy for the Captain's Bell to convert Kio into a Catian android: the androids are genetically based on humans so the changes needed are not very great.
They have devices that can change a human into a cat boy. The boy-girl transformation should be a breeze. So perhaps they (each individual) actually have a choice, and who would NOT want to be a Cat Girl?
in one of the episodes, Jen makes the assertion that Dogsians "have been on earth much longer than the Catians have". by their extent of involvement with the United States and Nato forces being in Russia for the missile launch, would it be safe to say that they pretty much propped up the US and Russia after WWII for the purpose of selling weapons technology and doing reasearch and development? it seems that she was able to manipulate the majority of world governments, and she all but outright states that Dogsians have basically been using and funding the United States as a tool for their ambitions for a considerable amount of time.
When Jens first sees the scrolls used by Aoi which was given to her by Ichika, she is at first utterly confused as to what it is. Then she discovers what they are, and utilizes it to her advantage in the last arc of the anime series. While Ichika does say that Jens acquired her scrolls on the black market (completely plausible given her connections), Jens probably wouldn't of considered using them at all had Ichika not of given them to Aoi in the first place (but yes, she wouldn't of been able to save Kio and Eris either). Otherwise one would think she would've utilized them right away, or at least knew about them as a backup plan in an earlier episode.