I tried making a 'vampire hunter/slayer', however, I misjudged how easily they can become friends+.
My sim met Vlad on his introduction visit (shortly after the 'neck' call). When he came knocking at the door, I had my sim go out to make him leave using mean interactions. As soon as the door was opened however, Vlad swept by him without so much as a word between them. My sim went to bed while Vlad played with everything in the house all night. When he left at 5 am with the 'Thanks for hanging out with me', I noticed in the relationship panel that they were now friends.
Caleb was a co-worker at his job (Detective) and since Lilith always visited Caleb at work, my sim got to know both of them, basically through job tasks. When they started showing up wherever my sim was when he went out, it was hard to notice that the relationship was growing with Caleb, but then one night I did notice my sim was always seeking out Caleb to talk to. Sure enough, the romance bar was growing. It went up quickly from there.
So much for being a 'vampire hunter/slayer'. My Sim and Caleb are now 'Married with Children'!