Flolo,
Now that you have all the advice about repairing the damage (to the extent possible), I'll throw a monkey wrench into the works:
Cut off the whole branch. It's in a rotten location. First, it's growing from the inside of a curve on the trunk (or very nearly) which is usually considered poor design, and second it's very nearly forming what's called a "bar branch" by being almost directly across from the branch on the right side of the tree. Bar branches are also considered poor design. The right branch grows on the outside of the curve (which is correct), so of the two that would be the one to keep.
Now having said all that, this is your tree and you should do what you want to do. The one reason I can see for keeping the left branch in spite of its flaws is that there is nothing else on that side of the tree right now until you reach the upper levels. You could really use a branch on the outside of the next curve up, but to get that you'd either have to have a very cooperative tree or you'd have to graft a branch on there. I've never done a graft, so I can't help with that.
Actually, there is perhaps one other possibility. You could maybe wire the foliage on the upper left so that it sweeps downward. You might be able to develop it so as to fill in some space on the left without having lower branches.
Hmmm . . . well . . . read up on bonsai styles and then do what you want.
