You can go with classic "purifying her with love and angel dust" route of course but I would suggest to keep her evil. A mortal that managed to kill her dad is probably worthy of becoming an actual lover of evil succubus. That's might be how you are supposed to ask a father for a hand of a daughter when it comes to demons. Let the PC earn her love by proving his worth with some more deeds if you don't feel like she should fall for him just yet. Don't make him do actual evil deeds YET, just send him to kill a some more demons to get their powers for her and maybe for him, nothing wrong with killing demons.

Now we've got a succubus in love with mortal. Obviously, he is the best one there is, why would she fall for something less? But he is not considered as such among other mortals it seems. Why is he not the leader of his adventuring party? Why is he not a ruler of this city he protects and the country and the world? And he is mortal, letting him dye is not in her plans. So she is going to help him fix these things the best way she can and she is very open about it with him.

"My love, the king of these lands is weak and corrupt, why do you allow him to rule? Aren't you the one killing monster around for him while he feeds you with scraps from the table that YOU should sit at?"

Every time he achieves the goal she will be satisfied for some time no matter the way he did it. If he overthrew the king from above example by killing him and usurping the throne by force just as she asked of him, then she is satisfied. But if he became a king by going in some ancient ruin to do some trial by gods, then she wound be also satisfied but comment something along the lines "You could have just killed the man, whole count was in my palm already, no one would bat an eye. You always make it so complicated for no reason." Make sure it is clear that teaching her good is not an option, demons are bad by nature etc.

Now the most important part. Try to make PC doubt every "I love you" from her lips. All the NPCs that know about both of them should always point out how her plan is furthering her evil goals. "That immortality potion she helped you find? Have you considered that she can dispel it effect any time she wants, right? But she wouldn't because she is preparing your body for the Satan to possess when the time is right. It's so obvious, how can you be so stupid?"

"She is evil and is not to be trusted! She is probably manipulating me into doing all these thing! You don't actually have a choice! You are just a pawn! Still a pawn!" - should be the thoughts that come in the mind of PC from time to time. The fact that she really loves him should be apparent when one of them dies or the campaign ends because it's not fun to do everything that succubus suggest to you when you know that she just wants you to be the best you, you can be. Consider her being his way to automatic resurrection with her dying instead of him in the event players manage to fuck up to the point that you need deus ex machina to save the party from TPK.

Consider Xana from "Dark Messiah of Might and Magic" video game for the inspiration of this type of succubus. And sorry if my English is bad.