We often spend a lot of time talking about the defensive handgun, carrying extra ammo and choosing the right kind of defensive holster. In the past few years, we've even begun to realize the value of always carrying some sort of tactical light with us. But we often overlook the value of carrying something that used to be found in every kid's pants pocket—a pocket knife.
Now, I will say right up front I have never had to defend myself with a knife, although that is certainly one of the uses that a good knife can be put to. But I have used one to cut seat belts in order to get an injured person out of a wrecked car. I have used them to cut wood shavings to start an emergency campfire and to clean the game that was roasted over that campfire. They can be used to cut makeshift splints to tie up a broken arm or leg. And when I was an active Sheriff one of my jailers used one to cut down a prisoner who attempted suicide by hanging himself.
You don't have to have a fixed-blade knife or a dedicated fighting knife. A general-utility folding knife will work just fine, although I would suggest getting a lock-blade knife so it doesn't fold up on your fingers under heavy use. And, just like with other defensive equipment, I would suggest you buy the best quality that you can possibly afford. Good steel stays sharp, and a dull knife just isn't much good for anything. Or you might prefer a Swiss Army knife or one of the many utility tools available, since they both have lots of extra gadgets on them. Not a problem.
In my youth, for a kid to get his first pocket knife was a rite of passage. It meant that grownups trusted you. To this day, when I meet a responsible kid, and I have his parents permission, I give him/her a pocket knife. Besides being a symbol of trust, they are just handy for hundreds of uses. It could even be that this is why someone first put pockets on pants....so he would have a handy place to carry his pocket knife.