Kotobuki can be nice trees. Even with all my experience with regular JBP I've not figured them out totally. Seems like you treat them more like a white pine. But, what you have there is a young grafted tree. The graft will almost certainly always be a problem, so plant that one in your yard as a landscape tree and find a tree to use for bonsai that you wont regret working on after you learn more. There's no part of a bonsai that you get to hide, and a glaring graft right at the base is going to disrupt any pleasing visual you manage to create.