Most anime series have that particular character that is really obsessed with defeating the main protagonist. In One-Punch Man, this archetype is filled by Speed-o’-Sound Sonic. Just as his name implies, he is one of the fastest guys around, and he happens to be a ninja. In his obsessive drive to defeat Saitama, he's shown that he can go through anything just to achieve success.
Speed-o’-Sound Sonic, commonly known as Sonic, was harshly trained in the Ninja Village to be an assassin. Sonic underwent the same training as the S-Class hero Flashy Flash, and even graduated from the same class. This means he could be just as powerful and formidable as Flashy Flash, able to surprise the hero several times with his speed during their spa, thanks to his encounter with Saitama.
Sonic is slim, but much stronger tan he appears. He also has a vast number of ninja weapons and explosives which he uses in fights. ONE has also insisted that a fight between Genos and Sonic would end in a draw, and he also said that Sonic would have won in his encounter with the Deep Sea King if he had his weapons with him. The Deep Sea King was a huge threat who ended every hero that confronted him, including some S-Class heroes, so if Sonic is powerful enough to defeat him, there’s not much need in emphasizing how tough he is.
Sonic was defeated by Saitama when they first met. This defeat gave him nightmares about Saitama, which drove him to train harder so he could defeat the hero and finally have peace of mind. Unfortunately, this won't be happening anytime soon. Even though Sonic knows this, he still won’t give up and has even gone the extra mile to obtain and eat the Monster cell just so he could achieve this dream... though that only caused stomach troubles.
Sonic has been unable to best Saitama in a fight, but if he wanted an easy victory, he could beat Saitama in a popularity contest. Considering how Saitama is always being underrated by other heroes due to his low ranking, Sonic could likely utilize this flaw to best Saitama in the Hero Association. He only needs to show-off his flashy skills and abilities while informing them of his achievement in the hero examination. Considering his skills and abilities are on par with Genos and almost at Flashy Flash’s level, the Hero Association wouldn’t think twice before offering him an S-Class certificate.
From the moment Saitama joined the Hero Association, he has always struggled to rise up the ranks and get his own fan club, and now Sonic has got just the right heart-wrenching news for Saitama that he got an S-Class certificate easily in the hero exam. Sonic could even take advantage of the influence associated with his hero rank to gain way more popularity and fan clubs than Saitama could ever achieve. Saitama might defeat countless monsters, but Sonic only requires one public and flashy win over a monster to gain popularity and an increased rank. This is one way Sonic could, in a sense, defeat Saitama.
To some extent, Sonic is just as fast and strong as Flashy Flash, and even faster than Genos. He definitely won’t have any issue being one of the strongest heroes and keeping his rank up in the hero world. Since Saitama still doesn't get the recognition he deserves, a popular contest is the only way Sonic can best Saitama.