Waddles
Mabel is very protective of Waddles, but she's not the only one. Mabel, Stan, Dipper and Soos all go on a rescue mission to save him in Land Before Swine. Also, in Gideon Rises, the four of them only blow their cover when they see Waddles being abused by Gideon. Waddles seems to love Mabel as much as she loves him, as shown on several occasions, including Gideon Rises.
Then there is that whole theory of "Waddles knows something," which is put to the test in Little Gift Shop of Horrors - Abaconings. I don't think it's plausible.
Blubs and Durland
It's very probable that either Durland is secretly a girl, or they're gay. Let me explain. The cops seem to be unusually close with each other, and Blubs often comments on Durland's qualities being endearing. In Irrational Treasure, at the museum, Blubs gets a blue balloon (the color given to Dipper and associated with boys) and Durland wants a pink balloon (the one given to Mabel and associated with girls). In the same episode, we find out that Ben Franklin was secretly a woman, proving that this kind of trick is 100% possible.
In Gravity Falls TV Episode 2, we hear Durland saying "Who wants to go drink expired apple juice, and then kiss each other..." First, as also seen in Gideon Rises, when apple juice ferments, it turns alcoholic. Second, it proves my point that either Durland is a girl, or they're gay.
Toby Determined
According to Mabel in MGT Art, Toby is "always weirder than I remember." And I agree. We know less about him than we do about a lot of Gravity Falls characters, and what we do know about him is strange. For example, he makes out with a cardboard cutout of his crush (Headhunters). He has the weirdest voice. And he has actual cat whiskers that the doctors can't explain (MGT Art).
Toby Determined is a failure at everything, it seems. He can't get Shandra Jimenez to notice him. He can't run a successful newspaper or even use real items (his microphone was a turkey baster and his camera was a cinder block.) And he failed miserably at being the Razz-Dazzler. The only thing he is good at is being in the Blind Eye Society, which seems counterintuitive since he runs a newspaper. Then again, it might be good. If the paper reported Gravity Falls weirdness, the Blind Eye Society's cover could be blown.
Carla "Hotpants" McCorkle
Someone with the name Carla is first mentioned in The Hand That Rocks the Mabel, in Gideon's commercial when Bud says "Carla, I've always loved you but I've never had the guts to say it."
Mrs. Gleeful is first seen in Little Dipper. She is seen as terrified of her son and possibly traumatized. This leads some fans, including me, to believe that she is Carla McCorkle and Bud Gleeful is Thistle Downe. Maybe Thistle/Bud forced Carla to marry him. This would further fuel the rivalry between Gideon and Stan, as Stan's ex-girlfriend gave birth to his worst enemy because she was tricked.
Jason the Genius
A picture was leaked sometime during the interval between seasons of Dipper's new friend, Jason the Genius. Jason is named after Dipper's voice actor and looks like a stereotypical nerd.
Candip
Some fans believe Candy has a crush on Dipper. I thought this was ridiculous at first, but there might be something to it. For example, in Summerween, when the Trickster threatens the kids with Gorney's fate, Candy instinctively jumps over and hugs the closest person, Dipper.
More evidence comes from Carpet Diem. Immediately after finding out about the electron carpet, Candy decides to switch bodies with Dipper.
In MGT Color, Candy displays quick-thinking ingenuity by showing off her picture, and Dipper enthusiastically agrees. This is probably the weakest evidence.
Obviously, Alex knows about this ship. He put Dipper and Candy in the "future matches" pile in The Love God.
Grenda
In MGT Fashion, Grenda says that in the next short, Mabel will talk about what next season has in store. She is not talking about Gravity Falls season 2; she is talking about fashion seasons.
According to Bipper, Grenda is a monster. It leads me to believe that even she might know something about Gravity Falls's secrets.
Pacifica
Is Pacifica involved with the mystery of Gravity Falls? Maybe. Her ancestor was an idiot, (or so we thought) but his descendants have turned his legacy around.
The reason Pacifica hates Mabel is because her weapon is her power and social standing. If Pacifica doesn't like you, you lose your social standing. Mabel neither has nor cares about social standing, so Pacifica doesn't have any power over her.
Mabel surprisingly lets Pacifica participate in her favorite color survey in MGT Color. This shows that Mabel doesn't hate Pacifica nearly as much as Pacifica hates Mabel, possibly because Mabel doesn't see Pacifica as a threat.
The dog that Mabel tries to milk in MGT Milking Things (MGT Art) apparently belongs to Pacifica.
The Golf War gives us a lot more insight into Pacifica's character. She's rich, and as Dipper says, "she's cheating at life." But at the end of the day, "she's just a normal kid." She's only different because of how much money she has. And it shows that she may not like her lifestyle. She might hate having to be "perfect" all the time. Update: after Northwest Mansion Mystery, we know all this to be 100% true.
President Sir Lord Quentin Trembley III, Esquire
Gravity Falls was founded by a man who was legally insane, so some of its laws are equally insane. For instance, woodpecker marriage is legal, finders keepers when it comes to property deeds (an experiment by Trembley in order to create a new kind of government called "gimmeocracy"), and a kid can legally be sent to ADULT JAIL.
Bobby Renzobbi
"Bobby Renzobbi once sold a really cheap tennis racket called "WHAT A RACKET!" which had overly tight strings that popped and blinded a few people resulting in lawsuits. He is always days away from prison."
Blendin Blandin
Blendin definitely appears in the three episodes that he went back to in the credits of the Time Traveler's Pig (Tourist Trapped, The Legend of the Gobblewonker, and Headhunters.) Some people say he appears in every episode of season 1, but I have yet to find him in any episode other than those four and Gideon Rises. (There, he makes a cameo during the scene when Stan drives up to the wreck of the Gideon-bot.) Oddly, we still don't know why he appears in Gideon Rises. One interesting thing is in that episode, he is still bald.
Blind Ivan / Toot Toot McBumbersnazzle
Blind Ivan has a cerebral map on his head. One of his tattoos is apparently misspelled, but the bigger issue is that at a different point in the episode, one of the journal pages shows a cerebral map, with one of the sections having a picture of Bill Cipher.
