Well I ain't got no girlfriend, hell I don't even know if this qualifies as a girl, but I do have an idea for who Darc's Vile Friend is! Say hello to [url=https://i.imgur.com/UpOXRj6.jpg]Cthylla[/url], Darc's mutual friend back when they were both still barely sentient slimes. Unlike Darc, Cythlla ended up tagging along with a new master to the Cradle Abyss. Unlike Darc's master who died an inglorious death at the hands of adventurers, Cythlla's master elevated themselves beyond humanity and has become one of the eldritch abominations that come from the Abyss. Cythlla herself has received a [i]massive[/i] power upgrade and is basically a dungeon lord herself, however she still acts in a servile role to her new master, hence her attire. 

Darc finds it strange that despite being levels above him and even their former master Cythlla still acts the part of a servant. However she gave Darc a handy lesson in handling power and authority and how sometimes it's easier and more efficient to operate behind the scenes acting as if you're just another pawn, instead of the true master mind. With that in mind Darc mainly sees Cythlla in order to figure out how he too can grow stronger and become a monstrous being like her, as well as give him important connections via the Unfathomably Unknown (Aka otherworldly knowledge) if he needs anything. For a price of course. In exchange she has vague plans that may involve Darc in order to spread the influence of her eldritch master, which Darc is A-Okay with.