well his npc business partner is in his early 40's, balding, overweight. has a some real particular things he enjoys - such as strip clubs. on the outside, he doesn't seem like much, but he's incredibly loyal and he's know fin for a long time. they work really well together but he (frank) is very rough around the edges. i imagine his sister is a smartass, spunky type who fin would also be pretty close with.
the ex would be someone he spent a certain amount of time hooking up with but fin has trouble committing much more than his body to a relationship. so it would have maybe been a few months and then she figured out he wasn't going to offer her more and went on her way. but when she shows back up, she has this kid that seems about the right age.
the younger person would be a fun, light light mostly. maybe a neighbor or someone he has contact with on a professional level. or just someone he sees in the same bar he visits that likes to flirt with him and tempt him to sleep with someone who he thinks is too young for him.