Most video game characters are designed to look cool or cute, and little else is put into their designs. Part of what I like about Knuckles is that the Sega Team really put a lot of thought into how Knuckles should look. From which animal to base him off of, to what color they chose to make him, Knuckles looks the way he does for a reason. I covered the echidna on another page; let's look at his other physical attributes.
Color - One of the most noticeable features in Knuckles is his red color. Red is prominent even on logos and emblems representing Knuckles. Echidnas aren't naturally red so there has to be a significance behind it. Religiously, red symbolizes blood and fire. Other symbolism leads it to signify sacrifice and courage. But are these attributes of Knuckles? Sort of. Sacrifice and blood go hand in hand, and when looking at Knuckles' character traits from a distance, sacrifice is a huge one. Knuckles sacrifices companionship, individuality, and any kind of life he might have for the duty of guarding the Master Emerald that has been bestowed upon him. He's forced to be alone for harmony and for peace. With his dedication to the Master Emerald comes a natural courage, and with that the symbolism fits completely. Also, Sega was looking to create a character to be guardian of the Master Emerald, and as such he had to be tough. Red is a color that fulfills that need, often associated with anger and wrath.
Face - His piercing eyes and striking facial features also help bring about that tough look Sega was aiming for when they made Knuckles. Knuckles rarely smiles. Instead, there's a noticeable weight in his face, carried with worry. Knuckles has a lot on him, and even though he appears tough, I bet given the opportunity he could break down. Knuckles won't let that happen, though. He has to stay tough for the Master Emerald, so he keeps it all inside.
Dreadlocks - Another noticeable feature is his dreadlocks. They point straight to the ground and rarely move unlike Sonic's quills. Such tight hair could be important because of his strong sense of duty, but that's stretching it kind of far. I'm going to opt for the fact that they just look cool, and that they're useful; he traps air under them allowing him to glide through the air, something no other character was able to do when he was first introduced in Sonic 3.
Knuckles - Now we're at the reason for his name. Knuckles has a mean punch, knocking villains out with the spikes on his knuckles as he glides to the ground. And knuckles he has; his hands are huge! He uses the spikes on his knuckles not only to fight but to cling onto surfaces, allowing him to climb walls to places Sonic wouldn't be able to get to. Another feature of his knuckles, added in the Sonic Adventure games, is his ability to dig through dirt to find the pieces of the shattered Master Emerald.
Shoes - Knuckles has huge shoes, a characteristic other characters in Sonic games share. His are unique to his character, though. One only needs to compare Knuckles' shoes to Sonic's and Tails' to see that. Sonic and Tails wear light pointed shoes, likely ideal for running. Knuckles' looks heavy and are shaped more square. This is likely due to the fact that Knuckles wasn't made to run. He's the guardian of the Master Emerald. He is made to fight. His shoes seem more suited for fighting than Sonic's. Knuckles' shoes even seem to have metal on them.
Like Sonic is known for his speed, Knuckles is known for his strength. He's the Power character in Sonic Heroes, after all. As such, Knuckles looks anything but cute. He has a rough exterior that contrasts who he is inside. His physical appearance makes him look hard and mean, and I don't think he would have it any other way.