Details for how to recruit Liara T’Soni.
Chronologically Liara is the last character you can recruit, and while the exact time when you recruit her is optional, you must end up recruiting her eventually. You’ll find Liara in the Prothean ruins on Therum, and she’ll automatically join your squad at the end of this mission. Therum is one of the game’s core missions, along with Feros, Noveria and Virmire. You can, in fact, complete the other three core missions before Therum, which will leave precious little game left by the time you recruit Liara - too little for a proper romance with her to develop. For that reason, we suggest you complete Therum first among the core missions, as you’ll have access to Liara’s services for longer.
You’ll unlock the Asari Ally achievement for completing five missions and/or assignment with Liara in your active party.
ME1: Complete 5 missions with the asari squad member