This page contains all the information you need on how to have a romance with Kaidan Alenko.
Kaidan is a romance option only available to Female Shepard if you chose to keep him alive in Mass Effect 1. Once you reunite during the mission, Priority: Mars, Kaiden 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 “How can you say that?” along with “I never left”.
Upon completing a mission, you’ll receive a message at your Private Terminal asking you to visit him. Here you’ll face a choice. If you didn’t romance anybody in Mass Effect 2 and remained loyal to him, 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 Kaiden is available here.
Meet with Kaidan
Complete another mission and then you’ll want to visit Kaidan at the Citadel to be informed that he has taken up the Spectre Job. During the mission, Priority: Citadel, you’ll find Kaidan escorting the Citadel Council. Here you’ll need Kaidan 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 Kaidan will trust you. However, if you don’t, then your other two options involve him dying. After the mission has come to a close and Kaidan has survived, meet with him at the Normandy Dock