I didn't come up with this, but I thought it was worth sharing.
So before Dragon Ball Z there was Dragon Ball. It had a lot of human characters that were skilled martial artists and (even though none were as strong as the alien Goku) they were able to hold their own in a fight. Once DBZ came around the human characters were quickly overpowered by all the space aliens, androids, and magic monsters that could destroy planets with energy blasts from their fingers. The amount of energy these people had got increasingly ridiculous to the point where the show was pretty much a shouting match followed by a "who can make the biggest lazer" contest. As a result the human characters either became comic relief or they were killed off to give the non-human heroes motivation to fight harder.
Essentially the humans became useless background characters with one major exception. Krillin was Goku's best friend and was with him through most of his adventures and battles. Most people think he is just some comic relief and occasionally moral support, but there are those who argue that he is the best fighter in the entire Dragon Ball franchise.
It's obvious to anyone who watches the show that Krillin is no where near the strongest fighter (not even close), but he manages to hold his own against seemingly impossibly overpowered opponents throughout the series.
Examples of Krillin Badassery:
- One Saibamen warrior obliterates Yamcha (who was about the same strength as Krillin) with one attack. Krillin manages to fight off three of them at once and destroy all three of them with one attack.
- After all the stronger fighters have thrown everything they have at Nappa he is still standing there unscratched. Krillin throws his special attack (Destructo Disk) at Nappa. Vegeta sees it coming and freaks out! Nappa realizes how dangerous the attack is just in time to escape with only a gash on his face. Had the attack landed it would have killed him for sure. Krillin was the weakest fighter left, but he came this close to taking out a guy at least 10 or 20 times as strong as him with one attack and was the only one to draw blood before Goku showed up.
- Later on Namek he's the only human (except bulma) on the planet and clearly the weakest fighter there. That doesn't stop him from holding his own against guys that are probably hundreds of time stronger than him. Yeah he get's his ass kicked, but he actually manages to put up a fight against impossible odds. He eventually dies, but only at the hands of Frieza (the most powerful dude in the univere). It wasn't some weak pointless death either. Krillin managed to cut off part of his Frieza's tail (more damage than Vegeta, Piccolo, and Gohan did combined). His death was so bad ass that it made Goku go Super Saiyan.
- He is resurrected later by Goku and all is good until the Androids show up. These things are way stronger than Frieza, but who is out there fighting them with the Super Saiyans... It's Krillin. The guy lands a few punches, and doesn't do much damage but can you blame him? These androids make super saiyan look like a joke. He's still only a human, but he doesn't use that as an excuse to stay home and watch the action on TV like everyone else.
- Flash forward to the Cell Games where Cell (who is a combination of the androids, Frieza, and all the strongest fighters) creates half a dozen "Cell Jrs". Goku, Trunks, and Vegeta have to go Super Saiyan (the form that made a fool of Frieza the most powerful creature in the universe) to beat these things. Krillin manages to fight one by himself and seems to be evenly matched. He only loses after getting sucker punched while checking up on Goku (who was getting his ass handed to him by a Cell Jr.) That means until he let his guard down, Krillin was fighting something that's easily as strong as a Super Saiyan. At this point Krillin, a human, manages to hold his own against a creature that a few years ago would have considered equal in strength to the most powerful creature in the universe.
- There's also this moment where Krillin attacks Buu (the most powerful creature in DBZ) and would have won had it not been for Buu's regeneration powers. Notice how Yamcha doesn't even bother to try to do anything. Krillin knows what he's up against but he still gives it his best shot. Buu totally doesn't see it coming, and had it been another villain without regeneration powers, his Destructo Disk would have saved the day.
Conclusion: Krillin is the greatest martial artist of all time, despite being one of the weakest fighters in DBZ. He was outmatched and underpowered from episode 1 of Dragon Ball Z. He could never be as powerful as the increasingly more god like beings he found himself surrounded by. In spite of this, he never quits or retires from action. The little guy goes into battle against things he knows he could never hope to defeat, but he never gives up and actually manages to hold up his own in a way that the other fighters would never be able to do if they found themselves at such a massive disadvantage. If he relied on powering up and firing a massive lazer every time like the others he would have been killed in a heartbeat, but instead he relies on his own skill and his brains to keep him alive and fighting.