This page contains all the information you need on how to have a romance with Kaidan Alenko.
Securing the Romance
Once you’ve completed the necessary steps to continuing the romance with Kaiden which can be found here, How To Start Kaiden Romance. From this point onwards, you’ll be able to speak with Kaiden aboard the Normandy and by doing so you’ll progress your relationship. Eventually, you’ll meet with Kaiden at the Presidium Commons which is located at the Citadel.
Here you’ll want to choose the following dialogue choices to secure your romance with Kaiden, “Glad we’re getting to talk” followed by “I want to be together”.
Finally, as you start the mission, Priority: Earth, Kaiden will meet you in your cabin and you’ll want to say the following before you can engage in an intimate moment together, “I’m glad you’re here”, “I love you” and “Then you’d better kiss me”.