Details for how to recruit - and how to retain the services of - Ashley Williams.
Ashley’s tenure as a squad member is almost as long as Kaiden’s; you’ll encounter her during your first mission on Eden Prime, as you make your way towards the Dig Site. While you can be adversarial with her the first time you speak (and subsequently) you can’t be sufficiently hostile as to run her off. Like Kaiden, she’ll remain a core component of your team until you manage to recruit another character on [the Citadel]. Only then can you bench her in favor of another character, should you so desire.
Also like Kaiden, if you want to keep Ashley around until the end of the game you’ll have to choose to rescue her on [Virmire] when you’re given a choice between relieving Ashley or Kaiden. You can find out more about this choice by following the link - beware of SPOILERS.
You’ll unlock the Soldier Ally achievement for completing five missions and/or assignment with Ashley in your active party.
ME1: Complete 5 missions with the Alliance soldier squad member