When it's Haré + Guu!
The boy is Haré, a ten-year-old who lives in the jungle with his mother. Guu, the girl, comes into their home to live as she has no parents. Her appearance is cute and sweet, very kawaii. They play together, then go to sleep. Next morning, Haré finds that Guu has changed drastically:
Her friendly, vivacious nature is now dour, ominous and flat. And she eats things. whole bananas. People. Animals. Perhaps eats is incorrect. She swallows them, and they enter a strange Wackyland
world inside.

There is a couple, a weird multi-legged cat, a menagerie of animals, all of which Guu can hwarf up when she chooses, with no harm beyond a time discontinuity. Guu is Not As She Seems, a liminal creature, an intermediate being.
Hilarity ensues, as Haré proceeds to stress over protecting his little world, family and friends against Guu's uncertain intentions. He is a little boy carrying a heavy weight. Guu is the trickster, bringing chaos into the village, and into Haré's life.
I bought this set back in aught-six, as I recall, from the producers of the English dub, AN. I watched a couple of episodes, then became distracted by busy-ness, or stuff, or ooh, look, a KITTY!
My shirt-printing life has shifted to the after-six PM-til-dawn mode, and so I have lots of time to be entertained whilst printing, with a pistol by my side. Yeppers, lookit ME, I'm trendy! (No I do not carry it into Publix, pointing out good prices with my laser sight...I am many things, but yahoo is not one of them. Neither am I a Houyhnhnm, but that's a horse of a different color.) I decided to give the series another try. Very happy with the results.
Haré's mom left (her very wealthy) home, pregnant by an unrevealed man and rejected by her father. She winds up on the jungle island, gives birth and raises Haré. The community welcomes them. I am currently halfway through the series, and will not do spoilers. If you enjoy bizarre shows, wacky, and off-beat, you may well enjoy Haré + Guu.
But stay away from vore. That's just creepy.