While Goku is Dragon Ball's main protagonist, it could be argued that other characters fit the profile as well. Bulma has been there since day one and is consistently useful in helping the gang with her technological skills. Other characters, however, have been rather lacking in terms of fighting capabilities and usefulness since their debut. One such character is Yamcha.
One of the few humans of the group, Yamcha was introduced as a "bad boy" in the early days of the original Dragon Ball series. He was a desert bandit who was tough on the outside, but terrified of talking to women on the inside. His Wolf Fang Fist attack was said to his deadliest technique despite him only being 16 at the time of his introduction.
After befriending Goku and the others, he and Bulma began a rocky relationship that ended during the three years before the Saiyans arrived on Earth. By this time, he had trained under Master Roshi and even learned the Kamehameha, which was only taught to Goku before him. He had also created several new Wolf Fang Fist techniques, like the Blinding Wolf Fang Fist and the Neo Wolf Fang Fist, when he was still young and entering his first World Martial Arts Tournament with Goku and Krillin. He had a near-encyclopedic knowledge of other fighters, including Grandpa Gohan. He would later train under King Kai after his death during the battle against the Saiyans.
It seemed like Yamcha had the skills to be a competent fighter, but after the Cell Saga, he lost the spark he had for fighting when he realized the Super Saiyans were superior. He gave up because someone was better at fighting than he was. Before, he was willing to put his life on the line to fight, even if he knew the odds were against him. Now, he stands on the sidelines and watches. Supporting friends is important, but doing what you love even if you're not the best is just as equally important. Tien is a great example of someone who keeps fighting despite knowing that the Saiyans are better -- because he simply loves to fight.
Poor Yamcha even lost in romance when he and Bulma broke up for good just before the Saiyans arrived, despite the fact that he still loved her. He then had to go on and watch as she met and married one of the people who tried to destroy the planet and contributed to his death. Not only that, but he had to watch as Vegeta went on to have two children with Bulma. While Yamcha and Bulma were still friends, he still had romantic feelings for her and watching her go on to have a happy marriage must have been heartbreaking for him. We even see some of this resentment in the way he talks to Vegeta.
Many of these shortcomings can be attributed to Akira Toriyama wanting to end the series after the Cell Saga and not knowing what to do with this large cast of characters in the following story arcs. Yamcha was just one of several characters to be pushed to the sidelines while the Saiyans took center stage due to their versatility. Yamcha has limitations as a human, but he's also supposed to be one of the most powerful humans in the world. He has the potential to do great things if just given the opportunity. He trained under the same masters as Goku, so that should count for something. Even if he doesn't have Saiyan DNA, he has the skill and potential to at least try and surpass his current level.