There's also speculation that Frankie is an Imaginary Friend that Madame Foster made based on herself when she was younger, due to the fact that nearly nothing about her past or personal life is shown or revealed.
Or similarly, that Madame Foster IS Frankie, time-traveled to the past.
Believe it or not, even Bendy is subject to this. Some people believe that his creator really did blame him for his own misdeeds, causing him toJump Off The Slippery Slope. This isn't entirely baseless; at the beginning, when his family's giving him up to Foster's, the kid is in the typical "I know I messed up and I'm ashamed" pose.
Bloo. Some fans find him very entertaining and endearing because of his antics, while other fans find him as an obnoxious jerkass who doesn't deserve Mac as a friend. Many fans have started to dislike Bloo because of his rude, selfish, and egotistical personality, and to make it more insulting, he got worse as the show went on. Just look at the stream of nothing but negative traits on his character page. So many fans kept wondering as to why Mac didn't just ditch Bloo later on. And you know it's bad when the deuteragonist is loathed by the fandom and the other characters.
Goo. There are some who see her as an adorable and funny character and those who see her as obnoxious and annoying. It doesn't help that her debut episode "Go Goo Go" garnered as much negative reaction as it gained positive.
Canon Fodder: Wilt. His Backstory wasn't revealed until the special "Good Wilt Hunting", spawning many, manyFanfics that provided their own explanation for his scars and history, often treading into Darker and Edgier territory than the actual series.
Crazy Awesome: Madame Foster. Worried you'll get in trouble for starting a party in the house? You sure will for not having her along.
Case in point, she's left to deal with a bear. Next time we see her she's riding the bear naked!
Cheese became this for many fans. Ironically, the reason Cheese became a Creator's Pet is because he was immensely popular with the fans in his debut episode. You know what they say; there is such a thing as too much, er, cheese.
Bloo also started to become this for a few fans after a while.
Family-Unfriendly Aesop: Bye Bye Nerdy is a dreadful episode to watch for this reason. Bloo freaks out that his creator is a nerd. Mac takes the correct stance - he doesn't care whether he's cool or not. Cue "Jamez Withazee", the self-designated coolest kid who thinks this makes Mac cool. What happens for the majority of the episode is the self-deprecation of Mac, who now only cares what "Jamez" thinks. However, neither character nor viewer realize until the end that "Jamez" horrible worldview of thinking that not caring is cool, extends to other people's well-being as well. Mac comes to his senses, but not before being mocked by "Jamez" and everyone else for being uncool. The asshat and his admirers are never called out on this.
Fanon Discontinuity: There are several fans who don't consider anything past season 1 as canon. To a lesser extent, there are those who won't accept anything past the three episode pilot.
The show's attempts at Black Comedy and/or Kafka Komedy ("Foster's Goes to Europe", "I Only Have Surprise For You", "Imposter's Home For Um... Make Em Up Pals" and the infamous "Everyone Knows It's Bendy") usually resulted in this.
"Funny Aneurysm" Moment: The scene where Wilt freaks out some kids by pulling a fake arm off his nub arm is pretty funny the first time through. Once we learn his backstory though ( specifically that he had to get the arm amputated after it was crushed beyond repair.) it becomes much darker. Not to mention a rather dark instance of Foreshadowing.
Hilarious in Hindsight: "World Wide Wabbit", which sees footage of Mr. Herriman's dancing for Madame Foster being uploaded to the internet, seems to be a spoof of any online videos going viral and becoming famous by the public for it...Until you realise that the episode premiered before YouTube even existed.
In the episode "Bye Bye Nerdy", Bloo is shown playing a Space Invaders-esque game with a power-up that shoots a giant vertical laser. A few years later a game title Space Invaders Extreme 2 has that exact same power-up.
In the episode "Seeing Red", Terrence imagines a pizza friend whom he eats, right after it says that it loves him, much to its horror. Cue twelve years later, Sausage Party gets released, which is a film about anthrophormorphic foods who revere humans at first but eventually learn that the humans they look up to use them for eating and not for friendship, once they get purchased.
Dutchess gets this for the same reason as Cheese, being detested in-universe and out.
Terrence, Mac's Big Brother Bully, isn't liked very much by the fandom either because he bullies Mac and he has no redeeming qualities whatsoever. Even Tara Strong went so far as to say that he was her least favorite character she ever portrayed, mostly due to her vocal chords being strained while voicing him.
Eurotrish, primarily as an Ethnic Scrappy due to her design and constant off-key singing. She's also a rare In-Universe Scrappy since the people of Europe kicked her out of the continent for these reasons.
Some Anvils Need to Be Dropped: "Where There's A Wilt, There's A Way" carries the obvious message that feeling obligated to do people favors can and will land you into unwanted positions, and it doesn't hurt to say "no" to people.
Suspiciously Similar Song: During Bloo's movie, Wilt performs a song that sounds very similar to "All Star" by Smash Mouth. Including the words "Hey Now".
They Wasted a Perfectly Good Character: Okay, so "perfectly good" might be a tad bit of a stretch when talking about Terrence, but there are some fans who were pretty disappointed that Terrence began to appear less throughout the rest of the season, for they felt that he never got the chance to develop as a character to explain his horrible behavior, especially towards Mac.
And let's also not forget Mac's family in general. His mother rarely appears and his father is never seen nor mentioned one time. Seeing as how Mac is the main character of the show, you'd think that there would be a bigger background on his family life.
They Wasted a Perfectly Good Plot: In the episode Foster's Go To Europe, anyone else like if they actually went to Europe? Imagine Bloo offending the locals and the wacky hijinks they'll pull across the continent.
Ugly Cute: Some of the imaginary friends. Wilt, Eduardo, and Coco come to mind immediately.
Villain Decay: Duchess. She was really only a villainous character in the pilot. After that she just became snobby and apathetic to everyone around her.
World, the reality-changing friend inside the toy box's imaginary world in Destination Imagination, is probably the biggest woobie in the series. The majority of the events in the special happened because of his emotional instability.
Which only existed because he'd been locked up inside a trunk by his creator's parents for an extremely long time, where he could create absolutely anything he wanted...except other sentient beings to share it with. No wonder the little guy was an emotional timebomb.
To a certain extent, Goo, at least in her debut episode. Prior to meeting Mac, she had no human friends and created imaginary friends to keep her company. It's even a bit worse in hindsight because when she tells this to Mac, she says nobody likes her because she's a "big, fat weirdo", subtly implying she might have been bullied before. There's also the possibility that her parents aren't much help because they give her too much freedom and little to no guidance.
Red during the episode of Seeing Red.
Eduardo, especially in any instance when he's sad.