What best describes the relationship of insulin to glucagon?

Glucagon releases stored sugar, in an emergency. Insulin is what you inject so that your body can make use of sugar.

Glucagon works by releasing the sugar that is stored in the patient's liver very quickly. I would really only recommend using it on diabetics who have already passed out or whose blood sugar is too-low to eat anything. Otherwise, cake icing, Orange Juice, etc. are much easier and cheaper substitutes.

Insulin, on the other hand, lowers the blood sugar by allowing your body to make use of glucose, and turn it in to energy.

If you are wondering if you need to inject insulin to accout for the sugar that the Glucagon releases, I would recommend that you do not do so. If somebody was low-enough to need a Glucagon shot, their body is going to be burning extra calories to get back up-to-speed for a few hours, and they might go low again if you inject any more insulin. A few hours with high blood sugars will not even really hurt a diabetic, but a second low might.