Bad Goblin...
If you've been following the Alchemist story, then you probably know that storyline was the introduction of an Alchemist NPC that my group would later come to hire. Now some you might wrinkle your nose and say, "Wait a minute Warlock... Alchemists are crusty old men!"
Yes. This is true. But let's all agree that no one starts out as a crusty old man! You have to but some years under your belt and well... get crusty! So what we have here is a young girl who has a passion for alchemy and some skill with it. She's developing and learning. The plan (which always seems to go astray!) is that she will devolve and become more useful as the party is forced to deal with greater and greater challenges!
So... let's talk about what you really want to talk about. Why is she fucking a goblin? Well, being young and naive, and left in a town of miscreants while the party goes adventuring, she comes into possession of a goblin slave. Being more attuned to books and science, she doesn't have a good handle on keeping goblins in order and the thing is soon running amuck in her house and in her!!!
Of course, none of this is being helped along by a devious wizard living in the town who has an agenda all his own!!!