For personal defense have you considered a 4oz. container of pepper spray in a belt pouch? You could keep it in your backpack, but slip it on your belt (maybe under your jacket or shirt) when you need to "activate" your pack in an emergency.

I second the suggestion that you find room for some water and food. Perhaps a couple of energy bars or some trail mix. In my work messenger bag I carry a couple of the 2.5oz packets of fruit and nut mix put out by Backpacker's Pantry. You could probably throw together a homemade equivalent.