Wow dude, this is really nice!
I'm also a manga/anime artist, and this is definitely good.
You're really good at doing different perspectives and positions for your characters, but some of them do not look balanced or natural. Also, your feet look really tiny in some of them. Try to keep the toes and legs proportional with the rest of the foot, especially if you're trying to add a 3-dimensional effect.
Try some other themes besides strictly ninjas. Maybe something original.