Ronan Dex, a Satedan, whose homeworld was destroyed when it tried to fight back against the Wraith (evil space vampires made from a combination of a life sucking bug and humans.) He was a Runner for seven years before being rescued from that life by the Atlantis Expedition.
Teyla Emmagan, an Athosian, another of the Pegasus galaxy, whose world was culled by the Wraith shortly after the Expedition arrived in Atlantis. She and her people joined them, for various reasons, including supporting Atlantis with food from farming. She herself is an accomplished fighter, and she also has some of that funky Wraith DNA in her system.
Lieutenant Colonel(formerly Major) John Sheppard is the leader of the military arm of the Atlantis Expedition, and the leader of this four person team that represents Atlantis' primary staff. He's a military officer with a bit of a loose cannon streak, often doing what he feels is right over what he should do under a chain of command. He also has a screwy sense of humor, and does not do feelings very well at all.
Dr. Rodney McKay is a Canadian scientist, first introduced in Stargate SG-1 where he proceeded to be a complete asshat until he was later redeemed a bit into the character he grew as on Atlantis. He's snarky, assholish, and basically kinda like Twilight Sparkle from season 1, except turned up to eleven, and with a ton of added whining and moaning. He also has a heart of gold and cares deeply for his fellow team members, and for people in general, even if most of the time he'll be a douchecanoe towards you due to his arrogance and intelligence.