Not every encounter or engagement is going to be a static target 25 yards away. Most attacks happen within what’s called the combatives bubble. If you were to draw a circle around yourself that was 7 yards from your centerline, that is your combatives bubble. Anything that happens within this 7 yards will more likely than not happen too quickly for you to present your primary weapon (your pistol). In this case you need to have a backup plan and that plan typically is a blade.
Now that we have determined that your secondary weapon is a blade, we could go down a rat hole for hours about what type of blade, type of steel, the design, and carry options. I recently wrote up an article about a Navy SEAL who ended up dispatching a pirate using his blade. In that article I discuss the reasoning behind a fixed blade over a folding knife.
When seconds count you want a fixed blade. Folding knives are great to have for everyday carry stuff, but when it comes to a secondary weapon for defense I want a fixed blade that I can easily deploy and not have to worry about opening the knife (If it’s a folder) or the lock failing under stress. There are several companies in the industry that make concealable fixed blade knives for this purpose.
Take a look at the following video to see Kyle Defoor discuss using a blade in a close quarters combatives scenario.