This page contains all the information you need on how to have a romance with Ashley Williams.
Ashley is a romance option only available to Male Shepard if you chose to keep her alive in Mass Effect 1. Once you reunite during the mission, Priority: Mars, Ashley will have questions about you and Cerberus. Here you’ll want to say “It was a temporary alliance” followed by “You know me better than that” the first time he brings this up, then when he brings it up again say “You think I’m like him?” and “I’m still that guy”.
Upon completing a mission, you’ll receive a message at your Private Terminal asking you to visit her. Here you’ll face a choice. If you didn’t romance anybody in Mass Effect 2 and remained loyal to her, then you’ll be able to reignite your relationship. If you did romance someone else in Mass Effect 2, then the opportunity to stay with Ashley is available here.
Meet with Ashley
Complete another mission and then you’ll want to visit Ashley at the Citadel to be informed that he has taken up the Spectre Job. During the mission, Priority: Citadel, you’ll find Ashley escorting the Citadel Council. Here you’ll need Ashley to trust you enough to side with you in order he survives this mission. This all depends on your actions throughout both Mass Effect 1 and 2.
If you have enough Reputation, then you can use a Charm or Intimidate interrupt and Ashley will trust you. However, if you don’t, then your other two options involve her dying. After the mission has come to a close and Ashley has survived, meet with her at the Normandy Dock