Actually, the brain can and does use ketones for enenergy, but it will always need approx. 100g of glucose that the body will derive from protein. Most of it seems actually to be needed for the eyes.
To an extent a ketodiet is more protein-sparing than being on a low-calorie higher carb diet. But once you are exercising hard, the rules are different again.
Still if you must be dumb and eat a very low calorie diet, while exercising lots, muscles would probably be better conserved with a ketodiet.. but only by a slim margin.
A smarter approach is to fuel high intensity effort in order to prevent muscle breakdown and use a (semi-)ketodiet on other days of the week.