A well balanced composition, I like the blue trees at the back and the blue shade you gave to Totoro as well as the mushrooms, texture and detail given to the bark of the trees.
Of course, this is not super original since it is Totoro, and I'm not sure if the commissioner asked for this pose and and setting, but I feel there are more interesting things Totoro could be doing. For example flying in the air with an umbrella in the night sky would be more exciting, or doing that rain dance.
Or, if Totoro is supposed to be in a homey, relaxed setting, perhaps lounging at the very top of a tree, to give it a more mysterious spin. However, I realize it may be your intention for just a typical simple, warm and friendly piece without all that mystery. It's just that I've always viewed Totoro as sort of mysterious, mystical being. I would have also accentuated more on Totoro's size and made the other mini-Totoros (sorry I don't know what they're called) smaller, to emphasize that.
Overall, it's a nice cute and simple piece with bright colours and all together pretty visually appealing. I was actually pretty surprise you'd request a critique on this, as I've said earlier, this just seemed like a simple piece, and not something you would dwell on for too long. I still like it, it's just not super special. (HOPE WE CAN STILL BE FRIENDS AFTER THIS)
The composition of this piece is astounding when you take into consideration the time, effort and skill that went into this water colour painting.
While the subject matter is not your original design, the way in which you chose to depict Totoro in water colour paint is a stroke of genius. The balance and blending of colour suggest a deep understanding of colour theory and how to use water colour paints effectively. I enjoy the magical and bright feeling the colours you chose, suggest.
My only concern is with the size of Totoro in relation to the other forest spirits/mini totoro's. In the film he is depicted as being massive compared to the two young girls in the movie, and those mini totoro's are smaller then the two girls.
Overall the piece is stunning, and I loved that you recorded yourself painting it. Great Job, keep up the great work.