What defines a fighter is going to be a completely relativistic, not to mention semantic, question. When you get right down to it, a guy can call anything he wants a "fighter", even if it has no true potential, as with fantasy/art knives.
At its most basic level, it's a knife that, well, one can fight with. This can range from a nail file, to a butcher knife, to a khukri, et al.
For functional fighting knives, the design is going to depend on the style being used as well.
As to the subject of guards, once again, this has to do with style and personal preference. I find it interesting that styles which are slash-heavy, such as most Asian styles and Finnish knife fighting, promote blades with minimal or non-existent guards. Always seemed a bit contradictory to me, since slashes are what a guard is, in theory, going to protect your hand from. Then again, perhaps because they rely so much on slashing they know something about the futility of a guard that we do not.
That's a bold speculation, considering European swords progressed the evolution of the guard right up until edged weapons became almost irrelevant.
When talking about modern, Western fighters, Bowie DNA seems to be the common denominator, and even the guards on these beasts run the gamut. Some of the more ostentatious guards may have been based on theory and the province of city slickers, but I know of at least one authority of repute who swears he can make them work, every time. That is, for their intended function of catching/turning/disarming blades. That's a bit beyond my skill set, and I'm happy with a guard that offers some small degree of protection, keeps my hand from slipping forward during a heavy thrust, and doesn't snag on clothing or equipment.
To this end, and for my individual purposes and tastes, this is my defining fighter.
Rob Patton/Kasper Grande. 8" blade, L6, nickle silver, G10.
At its most basic level, it's a knife that, well, one can fight with. This can range from a nail file, to a butcher knife, to a khukri, et al.
For functional fighting knives, the design is going to depend on the style being used as well.
As to the subject of guards, once again, this has to do with style and personal preference. I find it interesting that styles which are slash-heavy, such as most Asian styles and Finnish knife fighting, promote blades with minimal or non-existent guards. Always seemed a bit contradictory to me, since slashes are what a guard is, in theory, going to protect your hand from. Then again, perhaps because they rely so much on slashing they know something about the futility of a guard that we do not.
That's a bold speculation, considering European swords progressed the evolution of the guard right up until edged weapons became almost irrelevant.
When talking about modern, Western fighters, Bowie DNA seems to be the common denominator, and even the guards on these beasts run the gamut. Some of the more ostentatious guards may have been based on theory and the province of city slickers, but I know of at least one authority of repute who swears he can make them work, every time. That is, for their intended function of catching/turning/disarming blades. That's a bit beyond my skill set, and I'm happy with a guard that offers some small degree of protection, keeps my hand from slipping forward during a heavy thrust, and doesn't snag on clothing or equipment.
To this end, and for my individual purposes and tastes, this is my defining fighter.
Rob Patton/Kasper Grande. 8" blade, L6, nickle silver, G10.
, Don's(DH3) guardless fighters. 
