So... this would be a quite acceptable and nice class (with the exception of forgotten trick) if it were not for the fact that it would completely replace the rogue in any game where it was allowed. Essentially, it IS a rogue, with a whole bloody cheese on top of it. I do think that the rogue would deserve and need that cheese, but as an alternative to rogue, this is simply ... bad.
So, what would be the options to make this right? Well, it could be an archetype, and all ninja tricks could become rogue talents. This would have worked well if it was not for the addition of the pool mechanic (which I think is a good mechanic though, but problematic in an alternative class). As I saw someone mention, the whole pool mechanic could also be fit into a talent, although this might be conceptually ... vague for rogues in general. Another option is to remake it into a base class, and take many steps away from the rogue. This might be preferable, but as to how the class would actually look then, I do not know. Lastly, it could be kept as a rogue alternative, but nerfed (balanced in comparison to a rogue).
In the third case, I would suggest the removal of the forgotten trick trick (which is simply a very bad mechanic, since I do not see any ninja not taking it rather than any trick that seems remotely situational). I also believe that any ninja trick that does not need ki to function should be available to rogues as well (as a rogue talent). Light Steps should not be "given for free" (which it is compared to the rogue class), but I would rather suggest making it into a ninja trick that expends ki. Also, I am really not sure about ninja tricks that do consume ki which still allow one free usage per day, yet still being vastly superior to any known rogue talent. They should probably always expend ki in my opinion.
To be perfectly clear - the ninja is not unbalanced compared to other classes. That is not my point. It is, however, completely unbalanced compared to the class which it is said to be an alternative to. This is bad. I really hope the next iteration will have a lot of changes in some direction.