The lack of the How to Play file was the biggest issue for me. I had to do a couple of fast playthroughs to understand the game, but I more or less got it down now, so I'll write it for everyone: the Poppo grows once it hits enough trees, so be careful to balance your visits to the woods with the other locations. If you just keep alternating between the Cave and the Town you do keep it alive, but it won't "gain exp" so to speak. Remember: your mission is to make it grow, not to simply keep it alive.
As for the technical side: I LOVE the art style and the character design, it has a really peculiar touch to it and is more than enough to compensate for the pre-made backgrounds in the game, kudos to the artist!
On the other hand the audio is... lacking. Then again, it's written on your website that this game was made by just two people, so it's more than understandable.
Overall, once you get the gist of it, the game is actually enjoyable and unique in its genre. The idea of growing an animal that gradually goes out of control is a nice interpretation of the theme and the art style is quite something too! I really liked it and I do hope that this idea will be expanded in the future.