Even before Shepard docks on the Flotilla with Tali in her "Mass Effect 2" loyalty mission, it's easy to notice the tension; many fellow Quarians already treat your crew member as if she's already been convicted, and that does nothing to help rekindle her broken spirit. The only way Shepard can help is to clear Tali's name and restore her honor.
Speaking on Behalf of Tali
Head to the Migrant Fleet in the Vallhallan Threshold. Be sure to bring a squadmate who can use Overload, such as Miranda. You'll face plenty of the synthetic Geth during this mission, making Overload a very effective tool.
Before starting the trial, you can speak with a few of the Quarians lingering in the room. Most of them will have strong opinions, none of which can be swayed. Depending on Shepard's actions earlier in the game, Veetor and Kal'Raeegar may be present and support Tali; certain actions, such as submitting Veetor to Cerberus during the first mission, can lead to their loss of support, however.
After beginning the trial, Shepard represents Tali. After a surprising revelation comes up a few minutes into the cutscene, the board sends your party off to the Alarei to recover something invaluable.
A Pit Stop in Dangerous Quarters
All of your enemies on the Alarei are synthetic, so make use of Overload and similar abilities. Tali in particular can rewrite a Geth if you put points into her AI Hacking ability, giving you an extra soldier to help fight your battles for a brief period. This is especially useful on Hunters, dangerous Geth units that have the potential to cause major damage.
When you find Rael's body, make a run for it, even if there are enemies around; they'll disappear when you trigger the cutscene approaching the area with his body. With the research in hand, there's one final fight with a group of Geth led by a Geth Prime unit just up the stairs: Dispatch them using the same techniques used on earlier Geth units.
The Final Plea: Make it Count
Regardless of how the beginning of the trial went, this is where Shepard's words really count.
For Best Results: If you've earned a healthy number of Paragon or Renegade points, the Charm and Intimidate options will appear during Shepard's speech. Select these to prove Tali's undeniable loyalty to Quarians and their history. Tali becomes loyal after the board declares her innocence. The exact amount of points you'll need is unknown, and the outcome is the same whether you use Charm or Intimidate.
Regarding Evidence: No matter what you found on the Alarei, tell the board there was nothing; Shepard will earn Paragon points and sway the decision closer in Tali's favor. Like above, she'll become loyal once the board clears her name.
Get Help from Veetor and Kal'Raeegar: If you spoke to them both prior to the trial and both supported Tali, Shepard has a chance to lobby for their help. This action will also guarantee Tali's loyalty.
Consequences of Failure
If Tali gets convicted, you're not out of luck; she'll remain loyal to you after acknowledging she has no more ties left in the galaxy.
If you slander her father to clear Tali's name, you've lost your chance to earn Tali's loyalty. An email will come in from the Illusive Man warning you that she may not be thinking clearly enough to perform well during the final mission, which could have serious consequences throughout the series.
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