Tali'Zorah
Tali'Zorah nar Rayya (or simply Tali), voiced by Liz Sroka, is introduced in Mass Effect as a young quarian machinist and soon becomes a member of commander Shepard's squad.
According to Mass Effect 2 official website,
... renowned for her technical and engineering abilities, she is an important and respected member of the quarian fleet. Tali's unique insight allows her to hack mechanized opponents, overload shields, or deploy an automated Combat Drone.
The quarians
The quarians are humanoid race from the planet Rannoch, known for their skills with technology.
They created the geth, a species of rudimentary artificial intelligences, but became terrified of their own creations and tried to destroy them.
Ironically enough, as a result quarians' home planet was conquered by the geth, and quarians live onboard their starships. Their flotilla is called the Migrant Fleet. Ships are most valuable resource in quarian culture.
Quarians' immune system has atrophied almost completely after generations onboard the Migrant Fleet. As such, quarians wear special suits and do not remove them.
From Bioware Galactic Codex:
Some believe the quarians are cybernetic, a blend of machine and biology that can survive for a time in the cold vacuum of space. Others believe the quarians are simply so used to living on their substandard, makeshift ships that they never remove their survival suits.
Due to the weak immune system, the quarians are literally trapped inside their suits. Therefore Tali's face in never seen through the series.
In the game
During the events of the first game, Tali is on her Pilgrimage, quarian coming-of-age rite. She possesses information that can prove Saren Arterius' betrayal. Saren sends assassins after her, and once Shepard rescues her, she joins the player's party. It is later revealed that her father serves as an Admiral of Migrant Fleet.
In Mass Effect 2 Shepard encounters Tali again while investigating a Collector attack. She had completed her Pilgrimage, joined the crew of the Neema starship and earned the title Tali'Zorah vas Neema. However, after joining Shepard she is charged with treason by Admiralty Board.
From Mass Effect wikia.com:
Shepard learns that Tali has been accused of treason, and that she must return to the Flotilla to defend herself. Tali provides the current location of the Migrant Fleet. [...] Captain Danna reveals that the charges are that Tali smuggled active geth into the fleet. On the orders of her father, Admiral Rael'Zorah, Tali had been sending deactivated geth samples to the fleet. Tali made sure they were not dangerous.After this mission, her title changes to Tali'Zorah vas Normandy.
In Mass Effect 3, provided that Tali survived the previous game's final mission, she returns to quarian fleet and helps in the war against the geth. If she was not exiled from the fleet, she becomes an Admiral. Shepard then may choose to help the geth fully destroy the quarian fleet, in which case Tali commits suicide, or help the quarians to regain Rannoch, their lost home-world. In that case Tali rejoins the Normandy crew, although not as a playable character.