Paradise Killer

Straightforward. Spam One Last Kiss's Hangout dialog option until you get an achievement and the dialog is grayed and says maxed out. You can either Hangout every time you see them or simply wait 2-3 minutes in front of them and then speak again to allow you to choose the dialog again. The choices you make don't effect the outcome so you can just button mash through it. If they are going to give you a Probe option or a gift, they will regardless of how the Hangout conversations turn out.

One Last Kiss is the ghost of a character who was murdered, you can find them after the initial meeting in the Syndicate Cemetery area right by a fast travel after you get through the blood soaked temple area and solve a platform puzzle.