Gortash, who escapes the Hells after years of torture, and hires Karlach (a red tiefling) as his bodyguard because better the devil you know.

Karlach, late teens, orphaned and trying to find her place in the city, imprinting on Gortash (a thug from the Lower City, making his way up on the world) like a baby duckling, ignoring the parade of red flags.

Them working together well and getting along, despite opposite personalities and morals, because Gortash (who grew up unloved by his parents and abused by devils) enjoys being the object of Karlach's earnest, honest flattery and loyalty too much, and Karlach (poor street urchin with little to no formal education) sees Gortash scheme with politicians and criminals, design clockwork machines, and climb the social ladder with ease, and thinks the sun shines out of his ass.

If Gortash keeps the worst of his dealings hidden from Karlach, well, that's just good business sense. She's still young and green, she'll toughen up as she grows older and the reality of the world grinds her optimisim to dust. And if Karlach turns a blind eye to her boss' cruel streak, to the boxes of smuggled weapons coming in and out of the warehouses, and the dead bodies floating down the Chiontar, it's because they probably did something to deserve it, after all Gortash has been nothing but good to her.

Maybe Gortash starts to wonder if a little bit of goodwill could be leveraged to achieve more power. After all, a couple of kind words and small favours have worked wonders to ensure Karlach's loyalty. Maybe he can allow himself a kindness or two, ease the leash, raise the boot, just a smidge. Only for the harshness of reality to slap him in the face once again, the deal falls through, his enemies gain ground, he loses money and men, his plan pushed back, because he, for one instant, believed Karlach's rainbow coloured worldview.

So he tightens his grip on power even more, doubles down on his cruelty, and decides: Karlach must go. And when Zariel offers her deal, a soul for an ongoing supply of infernal steel, he sells her off without hesitation.