Wow... some wonderful words by Mr. M and Cass.
This is something I've mostly kept quiet ever since I first became a fan, but now that it's been brought up on this thread: I've always believed that Frankie is lonely and an outcast to some extent. She's friendly and sociable and gets along with nearly everyone, but she devotes literally all her free time living with beings that are considered by society as inferior, at the expense of a "real" life and job. That has to be seriously damaging to her social status and image.
While it's baffling to us that someone like her would be slighted in such a way, I do have some objective reasons why. Aside from the obvious stigma of being a glorified nanny-janitor to things that many deem as useless children's playthings, the tantrums and outbursts provoked by her job could potentially lead people to wrongly perceive her as an angry, hateful, bitter hag-of-a-girl. And I've noticed that in the ocassions where considerable attention is attracted from the outside world--the mall antics and subsequent ban in "Store Wars", the joyriding bus and cops in "Bus the Two of Us", the failed television exposure in "The Big Cheese"--that Frankie is often seen by others in a negative and uncomplimentary light. It all detracts from the wonderful, loving, big-hearted girl that she truly is.
And it seems as if Frankie isn't entirely satisfied with her physical appearance, at least as seen in that kitchen scene with Coco in "The Big Picture". This is especially true with how thin her legs are, which I've noticed is the exception instead of the rule in the Foster's universe. Many other adult female characters on the show(The dog owner's girlfriend/wife in "Who Let The Dogs In", Erin Peterson, the Deo television supermodels) are portrayed with normal-length waists and legs. That could make Frankie secretly insecure about the potential of her physical 'desirability' with same-age single males, especially in a modern youth culture obsessed with quick thrills details over depth and substance. Also, Frankie can as often be as insecure and shy as she is strong and independent; she was very nervous in her date with Dylan, she was sincerely hurt and saddened when Bendy called her a wannabe, etc.
It's indeed a tragic shame... but at the same time, they make me love her and care about her even more. And I'm sure that's true of everyone else here.
