Haré+Guu/WMG

Guu is omnipotent
Given the extent of her powers, it's not too much of a stretch.

Guu is some sort of Eldritch Abomination
Okay, this is all but canon, but someone might as well say it. For

Guu is really one of the forms of Nyarlatothep, the Eldritch Abomination.
For some reason, "she" is being punished by the Outer Gods by having to put up with humanity and improve the life of one little boy (maybe due to Dawn Summers). Guu is going to have a few laughs while going about it, though. Not only has she shown immense and strange powers, but her past is referred to as having been "configured".
 * Narly actually took this as a dare. Someone asked him "Okay, if you're so smart and powerful, can you actually improve a human's life by their own standards?" trying to use the old Free Will Logic Bomb. Now it can say "Yes, I did it once. I'm very aware of what I'm choosing to do to you."

Guu is an earlier attempt to create an Integrated Data Entity Agent.
The ability to rewrite matter and energy? Yep. Vague hints at another existence too alien for humans to comprehend? Oh yes. We even see both Agent personality templates in Guu; the Stepford Smiler and the Emotionless Girl. Granted, Guu herself is too alien and sarcastic to expose to Haruhi. :shudders at the thought:
 * Or maybe she's an exceptionally eccentric incarnation of Haruhi...

Guu is the 3rd Angel in the Rebuild timeline.
Angels can take human form, for example. Angels exist to bring about the end of humanity. Guu has the power to end humanity at her fingertips, or more appropriately in her stomach. Use your brain.
 * Alternately, Guu is the herald of the Spiral Nemesis.
 * Jossed, as Guu can't "end" humanity, anything she swallows remains alive.
 * You're assuming that this isn't the case because she happens to like it that way, for the moment.

Guu's stomach somehow taps into the same universe as Wackyland in the old Looney Tunes short Porky's Adventures in Wackyland, and we simply never see the characters from that short!
== The whole universe is living inside Guu's stomach and the Guu we know is only a slightly more human duplicate of herself that she swallowed too because she got bored of being the only thing in actual existence. ==

Guu is somehow related to Mandy

 * I mean come on! Same size, same noseless face, same stoic attitude, same ability to make a bitch out of even the most powerful people; not to mention her eyebrows looks like an "M" if you bridge the gap between them.

Guu CAUSED Dr. Clive to come work in the village.
The odds of probability are a funny thing. A VERY funny thing: two people who live in a town the size of Salem (Massachusits or Oregon) may meet only once while at a restaurant and may never see each other again, even if they lived in the same town their whole lives. Considering that Weda moved to an undisclosed location in a jungle where the distance between it and Weda's home town is never specified, one finds it quite a coincidence that Dr. Clive JUST so happens to move there only when his son JUST so happens to start asking about him. Besides, whatever work he had in the city was pretty steady for the past ten years for him to work in the Weda household, so I assume Clive made bank before moving to the village. So why is he working as a school nurse? My guess is Guu either: A. Peered into Weda's subconscious using some sort of Guu logic, found out about Dr Clive. Or: B. Used DNA testing from some of Hale's DNA to find out about Clive. After all, she has plenty of access to it. She then managed to, in one night, F*ck up Clive's fortune, business, and reputation enough that he would have no reason to return home, then she swallowed him and carried him back to the Jungle. She then spat him up in a pre-furnished office specifically built to fit his needs and specifications and then explained to him (In a different form, possibly her hot chick form) that this village will give him all wants (Weda) and all he needs (Weda again, or more specifically a family to teach him responsibility and compassion. Thenceforth she PRETENDED to not know who Dr. Clive was or his relation to Hale so that she wouldn't be arrested for all the accounts of fraud and possibly robbery/arson she committed to force Clive into his new way of life.
 * Guu? Arrested? What are you smoking?

This anime is set in the distant future of Avatar: The Last Airbender
First and foremost, most of the jungle animals are somewhat mix&match: (Pokute are Rabbit-leamurs, Those pink Birds are naked mole swallows, Hee was a Banana Canary (Hence why Guu thought both were disgusting) Ect.)In this very distant future (Maybe a millenia or two after Aang's death)Bending has become a rudimentury art, on par with opening a can of beer or flushing a toilet, therefore no one talks about it much or marvels over it. Most people in Hale's time can bend all four elements to a small degree, most have a greater control over the element that best matches their personality: Hale and Weda are probably Waterbenders (when Hale is partucularly upset or angry at something it begins to rain like crazy. Coincidence? Plus they did float out really far into the ocean on the beach episode, even for a rip-tide. My guess is they subconciecly waterbended away from land while asleep.),The crazy barber lady is an Earthbender (Episode 18. All I'm gonna say.) Plus at least one of the schoolkids for them to be able to dig such a big pit in episode 19 in such a short amount of time, and at least one of them is a powerful firebender (My guess is either Mari or the girl who told the story of the little match girl in episode 20.)Bending is now taught by parents in all four forms to at least a minor degree, to make rudimentury taskes such as digging and gathering water easier.

Last, but not least, Guu is the new Avatar(Bet you saw that coming from a mile away). Hale (And Dr Clive, to an extent), are the only ones unaware of this. When Guu first arrived in the village, she told everyone at the village meeting who and what she was and they accepted her with open arms. All the parents told their children of this development and thus almost no one in the village is alarmed at her antics. Weda didn't tell Hale about who Guu was because to do so would make him overly jealous of her and could cause unheard of damage to either his own mental state or Guu's physical one. Besides, Hale has an idealist image of what an Avatar should be like, so when he see's Guu, he thinks monster as opposed to Avatar. One more thing: Since the four elements are now practiced by everyone the Avatar is now charged with (As well as protecting the people and preventing another world war) discovering what other elements or principles can be bent by humans. Most of the crazy stuff that Guu does on the show is either practicing other bending tecniques developed by former Avatars or discovering and applying tecniques she developed herself.