In the Mass Effect universe, two particular squadmates can be counted as Commander Shepard's most loyal: Garrus Vakarian and Tali'Zorah.
Tali begins rather humbly. She appears early in the first Mass Effect game, wounded and on the run. Commander Shepard helps her get out of a sticky situation, then Tali returns the favor with some hard evidence on Saren Arterius, the rogue Spectre. Ever since, Tali has been instrumental in Shepard's quest to save the galaxy, one mission at a time.
The Pilgrimage
When Commander Shepard meets Tali on the Citadel, she is in the middle of a Pilgrimage, an important rite of passage among the Quarians. Such a rite doesn't just prove the young individual's worth; it's also a pragmatic mission to find something useful and return it to the Migrant Fleet. Tali hasn't yet found such a thing, but she did come across audio evidence of Saren's attack on Eden Prime. She arrives at the Citadel to trade it in for money and Fist the human mobster tells her to meet the Shadow Broker in the wards.
This is a lie; he's actually sending her to Saren, who is desperate to keep that audio file under wraps. Fortunately, Shepard and Garrus are both trying to take Saren dow, and the solution is to rescue Tali and obtain that evidence before Saren gets it. Once that is done, Tali presents that evidence to the Council and Saren's guilt is accepted as fact. Tali may then join Shepard's crew, both to help take down the Geth threat and complete her Pilgrimage.
During the mission, Tali provides insight on the Geth and the Quarian people alike, and she helps Adams keep the Normandy's engine running smoothly. She is fascinated by the Normandy, but it feels oddly quiet and sparsely populated compared to the ramshackle, crowded vessels of the Migrant Fleet. She also reveals that she is the daughter of the famed Admiral Rael'Zorah, so Tali is under a lot of pressure to succeed and prove herself. Rael'Zorah supports his daughter, though rather distantly.
Law And Order
Two years later, during the events of Mass Effect 2, the Illusive Man includes Tali on his list of dossiers for Shepard to look into. Garrus is another familiar face who's on board and now it's Tali's turn.
She is on the fringe world of Haestrom, working to collect evidence on a star that's dying unnaturally fast. The Geth have since muscled in and Shepard must fight to evacuate Tali from the world, then escort her to the Normandy-SR2. Once again, she helps maintain the engine, but soon enough, she receives a strange and highly distressing message from the Fleet. She is facing criminal charges of treason and must return home for a trial. Shepard flies her there and the truth is revealed.
For a time, Tali had been acting on her father's orders to bring back Geth pieces for study, but the pieces have since assembled themselves into functioning Geth platforms, threatening the fleet and capturing a ship. Tali and Shepard look into it and the horrible truth is that Rael'Zorah assembled those Geth on purpose, to practice fighting them in preparation for an upcoming invasion in Geth space.
Rael himself is dead and Tali is desperate to hide this truth, or else her entire family's name will be ruined. Whether or not Shepard reveals that fact to the court, Tali is most likely to have her name cleared. She goes on to help disable some key components in the Collector base during the final mission.
Homecoming
Tali joins Shepard's team once again in Mass Effect 3, helping to disable a mighty Geth dreadnought during an intense space battle. Tali is not terribly fond of the Quarian-instigated war against the Geth, but all the same, she is concerned about the Reaper threat and most Geth have allied with their Reaper superiors. At last, Tali sets foot on the Quarian homeworld of Rannoch and helps Shepard disable Geth bases and lay the groundwork for a local Reaper destroyer's demise.
Once the dust settles, Tali daydreams fondly about building her very own house on Rannoch's soil. In some cases, the Geth are there to help. It is difficult, but possible, for Shepard to reconcile the Geth and Quarians. If that happens, Tali will accept the Geth as full-fledged synthetic people with souls and become their friends. In other cases, Shepard must choose between the Geth and Quarians. If Shepard sides with the Geth, then Tali will remove her face mask to breathe Rannoch's air freely, then fling herself over a cliff to end her despair at the loss of her race.
Tali In The Game
In all three Mass Effect games, Tali fights as an Engineer, using tech abilities to disable her enemies and damage synthetics. As such, she favors shotguns and pistols. She can even hack synthetics and turn them against each other, creating a fourth squadmate of sorts.
In Mass Effect 2, she can use the Combat Drone ability, creating a sphere that can deal damage to enemies and (in some cases) explode upon death, dealing even more damage. The Sabotage ability is especially potent against synthetics, disabling their weapons and knocking their weapons offline.