Flex

Filleting is a matter of running a blade along bone. A flexible blade bends against the ribcage and the backbone so the edge stays in contact with the frame and leaves nothing behind, which is what gives you a clean fillet with no meat wasted.

Stiffer blades give you control and push through more. That suits small panfish where finesse matters less, bony fish where you are cutting through structure, and anyone who is not confident with a knife yet.

Blade length interacts with this. A six inch blade suits panfish and trout, seven to nine inches suits walleye and bass, and anything longer is for big fish.

Flex is not the same as flimsy. A good flexible blade springs back straight when you release it; a cheap one takes a set and stays bent, which makes every cut afterwards harder to control.