So I had a stray idea about Lilith running across someone in a bad way and the only way to save them she can come up with is turning them. Miracle they don't die along the way, but they don't. Anyway, so now they both have to deal with the obvious consequences of that. Now whoever it is will live forever (yay...) and has to sustain themselves with blood or at least creepy tree-fruit.
Of course, Lilith will stay in their life and teach them how to vampire, but bitterness may eventually ensue. Might get really awkward as their relationship gets complicated and things swing between "thanks I owe you" and "dammit you should've left me alone." Half the time Lilith's not sure what the right answer there actually was and probably sooner or later decides the answer is there was no right answer. I'm sure eventually things level out and their relationship does too, but in the meantime, It's Complicated.
Can be a new character, or an existing one, idm. Uhh, in our board canon, spellcasters and vampires can hybridise, one can be both, vampire-fish makes sense to me too so I wouldn't be opposed to that one either, but aliens have very different biology and they don't vampire.