Shevat was built atop Babel Tower early in the 9000 millennium, using technology salvaged from Zeboim and the Eldridge. Where these people came from is unclear - it seems likely they're descendants of the ruined Zeboim civilization, but in the past we've also speculated that they were somehow connected to Solaris. They have quite a lot of interesting technology for a bunch of land-dwellers.
They're set on recovering as much of the lost technology as they can and use it against Solaris. The origin of this enmity is kind of hazy, but I would guess it also has to do with their excavations in the Aquavy area - they just happened to be sitting on a gold mine, and I'm sure Solaris wants it all to themselves. This carried over into Nisan's rebellion, and Shevat's treasures (namely the Excalibur I and notes on nanotechnology) fell into Sophia's hands. I bet we could trace Kislev's industrial advancements back to this period of time as well.
Social Structure and Leadership
Shevat is ruled by Queen Zephyr and her advisors. The three sages (Melchoir, Gaspar, Balthasar) hold great influence over both the queen and the people, but only Gaspar is present on a regular basis. I read somewhere that Shevat was originally ruled by a sort of council or parliament, but I have no details on the structure of the government before Zephyr's reign.
It's unclear how many of her advisors are relics of the Solaris War in Episode IV, but we know for sure that Zephyr herself and all three of the sages are kept alive with Krelian's life-extending treatments. They seek to repent for Shevat's betrayal at that time, but the rest of her council seems unrepentant. And while she is their monarch, it looks like she's still subject to their vote on important matters.
The citizens do not share Solaris's prejudice against land-dwellers, despite the fact that their home is in the sky. They understand the need for new blood, and welcome any land-dwellers recruited to their cause.
Although only one telepath has come out of Shevat that we know of (Midori), that might be a special quirk of theirs. They also harbor the chuchu species (and if someone can supply their name, it'll be very nice) for some unknown reason. ^.^
I have the impression (from the game) that Shevat's military isn't very extensive. They reconstructed the Excalibur - creatively named 'Excalibur II' - and slapped together a few gear units, but Solaris is wearing them down little by little. They have a decently vast network of spies on the surface to keep an eye on the land-dwelling nations and seek out worthy recruits, but they can't be open about their activities. In that way, Krelian succeeded admirably in keeping them cut off from the rest of the world.
Personally, I think they should have expanded on Shevat's historical role more in the game. We get an idea of what it's like when we walk around, but as a country, I think it was under-developed. It deserves a few more cultural notes, but sadly, they only exist in the Deus Ex Machina universe. The characters, at least, were very interesting. ^.^