Teyla is a native of the world the team encounters on their first mission – a world once inhabited by the Ancients. In her short life (her age is between 20 and 25 years) she has lost most of her family to the powerful Wraith but has become a leader to her people.

Teyla joins the team in an effort to defeat the Wraith, now an enemy of Earth. Feelings develop between Teyla and Sheppard, with their budding romance becoming playful sexual tension.

Teyla is a strong fighter and a survivor. She proves herself to be an invaluable source of knowledge about the people and worlds the team will visit.

From Stargate Atlantis.com

Teyla Through the Show

In 2004, a security team from the Atlantis expedition comprising of Col. Marshall Sumner, Lt. Aiden Ford, and John Sheppard gated to Athos in order to find either an alternate Zero Point Module or a place to evacuate, as Atlantis was slowly losing its shield (which protected it from the pressures associated with being submerged in the sea).
Teyla took a liking to Sheppard, who did not “look through” her like Sumner had, and lead him to the Athosian’s ancient city. It is here that Sheppard found her necklace, which she had lost “long ago”, and Sheppard also learned more about the Wraith via drawings on the walls. Drawings that likely dated back 10,000 years or more.
But then, the Wraith attacked. Teyla saved Sheppard, but was captured herself, alongside several more of her people and Atlantian military personnel, including Col. Sumner. Sheppard and the survivors of the attack returned to Atlantis, where Sheppard organized a rescue team. Using a Puddle Jumper, Sheppard and Ford infiltrated the Wraith hiveship and rescued Teyla and the other hostages, except Col. Sumner whom Sheppard was forced to kill. (“Rising”)
While Teyla volunteered her services as a guide to the Atlantis expedition, she slowly lost the respect of some of her people. Some even believed that she wanted the Earth-Humans to be “the Ancestors” that are often spoken of in their folklore, thus allegedly blinding her judgment. This rift became apparent when USMC personnel, such as Sergeant Bates, began vocalizing their belief that the Athosians were a security threat. This judgment was not reversed by any means when it was discovered that Teyla may have been compromising Sheppard’s SG team.
Teyla was vindicated when it was discovered that the Wraith planted a homing device, activated by a person possessing the Ancient gene, in the necklace that Sheppard recovered. Using this knowledge, Sheppard and his team managed to trap the Wraith, recovering a Wraith commander dubbed “Steve”. However, she became separated from her people, who had decided to inhabit the main landmass on the planet Atlantis was on. (“Suspicion”)
Teyla later introduced Sheppard to a race called the Genii, a militaristic and fascist race bent on destroying the Wraith — although they projected a facade of being an Amish-type people to avert suspicion. One of her friends was a Genii woman named Sora, although this friendship was severely damaged when Sora accused Teyla of leaving Sora’s father to die on a Wraith hive-ship. (“Underground”)
Later, Teyla aided in the evacuation of her people from the mainland, when she and Sheppard happened upon a massive storm heading toward Atlantis. A majority of her people were relocated to another world, a world with close ties to the Genii, who were alerted that Atlantis was being evacuated. Teyla, Carson Beckett and Lt. Ford were trapped on the mainland, which helped for the best when a team comprised of Acastus Kolya and Sora invaded Atlantis. Fortunately, the Genii plans to take the C-4 and Atlantis were thwarted, although Sora (once realizing that Teyla refused to kill her in a fight and they they would both die outside the shielded control tower), aided Teyla in saving Beckett. Kolya escaped through the Stargate, shot by Sheppard. (“The Storm and The Eye”)
After recurring nightmares, it was discovered that Teyla’s ancestors were experimented on by a Wraith scientist. The scientist attempted to make humans a more palatable food-source by splicing Wraith DNA. While the scientist and most of its subjects were terminated by the Wraith, who viewed the experiments with disgust, Teyla also possessed some of the Wraith DNA. This explains why Teyla was able to detect the Wraith prior to their approach — and why Teyla believed there to be a Wraith on Atlantis. (“The Gift”)
Teyla later discovered that she could intercept Wraith telepathic communications, even from long distances. Guided by Dr. Beckett, she attempted to learn more about the Wraith’s intentions for Atlantis by taking control of one, though this was futile as the Wraith were stronger psionically and controlled her mind to use her to deliver a warning to the Atlantis expedition. This event galvanized the distrust Sgt. Bates had for Teyla; Bates went as so far as to insult her by claiming that she was an unwilling Wraith agent when the Alpha Site was overrun by Wraith a short time after she was possessed.
Teyla was later believed to have attacked Bates, until it was discovered that a Wraith scout who scanned Atlantis in “The Brotherhood” had transported into the city. “Bob” was later captured and killed by Sheppard, thus vindicating Teyla. (“The Siege, Part 1″)
Teyla later led a team of Athosians who wished to defend Atlantis, along with reinforcements from Stargate Command, as the Wraith began their assault. (“The Siege, Part 2″) Teyla escorted Rodney McKay and the ZPM in his possession to the power generation chamber when the two original guards were shot by a Wraith stunner. She was also instrumental in convincing the Wraith that the Humans were intent on destroying Atlantis via her telepathic abilities. This was part of a ruse to detonate a nuclear warhead above Atlantis and tie in a puddle jumper’s stealth generator; with the city cloaked, the Wraith left believing that the humans destroyed Atlantis. (“The Siege, Part 3″)
To be continued…..

Source: Wikipédia