Sebastian is a bit trickier since there isn’t a romance. At best, the Regent can become friends with benefits with him.
Spoilers. After you finish chapter one, Sebastian will contact you to go to the Wild Wheelie. Accept and go there. Be polite, don’t be a dick. If you can, avoid engaging with the three vampires who try to pick a fight with you. Engaging with them raises the risk of Sebastian getting a bad impression of the Regent if you pick the wrong choices.
If you answered “manipulating my opponents” at the beginning of the character creation, Sebastian will give you a unique dialogue tree. He’ll ask you about Alex, specifically if you loved Alex. Don’t choose the options where you can say you didn’t actually love Alex. Sebastian will hate your guts for it. When he asks you what’s wrong with him don’t call him a cry baby. The conversation will wrap up and Sebastian will eventually call you back.
When Sebastian calls you the second time, accompany him to Bite Bay and spend the night there. Pick either “I was hoping to have some fun now.” or “We can still salvage it.” and you will sleep with him. If you stay faithful to him throughout the story, he’ll admit that he doesn’t want to be without anyone but you in the Epilogue.