I am extremely happy to see an action-oriented series here on newgrounds. It is very rare to see it executed to this level, so great job! Now on to some criticisms:
1.) Character designs: I actually really enjoy the main character's design as well as the other animalistic ones. They seem well defined and have a recognizable silhouette, it seems to me that this is a style you are very comfortable with. The human design however seems uninspired. He looked like a typical anime sensei both in design and in character.
2.) Backgrounds: I think you are actually very good at this, but there were places were it seemed empty ALTHOUGH it was kinda by design. The plain looking area is the fighting area in front of the dojo, I understand that you needed the open space for the battle, but it is very bland looking. The forest and interiors look great though.
3.) Sprites: I saw in your update video that you're not gonna do a sprite-based fight scene again in the future, but I would take it a step further: get rid of the pixelated outlines all together. It's a detriment to the quality of the animation, you're not trying to fit it into a gameboy cartridge and clean lines would look a lot better. The pixelation is distracting and makes things look muddy, especially in the backgrounds, which is a shame because compositionally they're really good.
4.) Directing: The non-sprite based scenes I think are pretty decently directed. It's easy to follow the characters and the timing works with the conversations and actions. HOWEVER you did the budget-saving anime thing where the camera shows a mostly static environment when two characters are talking. This happened a couple times and it did not work. Again in your update video you mentioned that you're gonna be taking time to read cinematography and directing books so I don't believe this will be an issue in the future.
5.) Sound: I'm gonna include the voice acting in here too. The voices were not all at the same level. The sensei guy is much quieter than the protagonist and his delivery is not great. Not only is he difficult to understand sometimes, but it's so dry that it did not hold much interest. The tiger being pitch-shifted to have a deeper voice I don't think was a very good choice, he sounded muffled and again difficult to understand. The protagonists voice (while very generic anime protagonist sounding) works and I believe he had the best performance. The music was perfectly fine, however there were a few instances where there wasn't any and it made the characters' conversations drag on a bit.
6.) Story: I would argue that this is the most important part of any series or movie. Even if the art is the ugliest thing in the world the story would save it and make the viewer look past that. Your story is not bad, but unfortunately it is extremely generic. It appears to have the same "last of his kind", "hidden potential", "find X amount of thing", plot that A LOT of shonen anime have. This is going to be the biggest hurdle to get past because there is not much investment in the story since it's all stuff I've seen before.
Overall I think you are a very ambitious person and I applaud you for taking on such a momentous work. I think there is a lot to praise here, but there also is a lot to improve. I look forward to seeing what you do next.