Kelly is like a secret romantic choice in Mass Effect 2. Unlike other romantic options, there are no loyalty missions that players should complete, and there is no real comparison point in the development of the relationship. Kelly can be romanced regardless of the gender of the player character, and you can love Kelly without interfering with other romantic options.
The type of relationship you have with Kelly is actually set up from the first conversation you have with her. When you first talk to Kelly, select the “Are you happy here?” Option and then the “We won’t.” When Kelly talks about closing her eyes and letting Shepherd catch her, choose “I would hug you” and set the relationship on a romantic journey.
Eventually you will be able to invite Kelly to dinner and her Kelly will offer to take care of your fish. To ensure that this happens, you must always be positive in every interaction with Kelly. Finally, Kelly says she wants to spend more time with you, and you can choose the “Let’s go to dinner” option. Then you have to get Kelly over a suicide mission. After the mission, talk to her and select “Are you okay?”
The next time you leave and return to Normandy, you will receive a private message from Kelly from the terminal and you can invite her to your cabin. After the cabin scene, the romance is really closed.