Speed-o'-Sound Sonic/Relationships

Saitama
Speed-o'-Sound Sonic views Saitama as his eternal rival and vows to defeat him. Saitama views their one-sided rivalry relationship in a nonchalant manner. He doesn't mind Sonic's persistence and their continuous 'battles', even going as far as encouraging Sonic to do his best. However, he does grow irritated if Sonic starts to bother him during important activities or causes havoc around others.

According to Genos, Sonic is one of the few people who recognizes Saitama's true strength. This recognition is what caused Sonic to stop working and continue training in order to surpass Saitama. This recognition and dedication, as well as declaring Saitama a rival and admitting defeat at his hands, indicates (in his own way) a form of respect that Sonic has for Saitama.

After pretending to be Saitama for a day, Sonic gains a new respect for the hero, vowing to never attack Saitama while the hero is fighting monsters.

Genos
Due to his obsession with Saitama and constantly attacking him, Genos despises Speed-o'-Sound Sonic and attempts to eradicate him when he has the chance. He also considers Sonic a rival and a reason to become stronger.

Puri-Puri Prisoner
Sonic's relationship with Puri differs from his relationships with other heroes; while Sonic genuinely respects Puri's physical strength he otherwise thinks of him as a fool and a weirdo.

In an audiobook, Sonic is chased by Puri at a festival after the latter mistakes a challenge for an offer for sex.

Flashy Flash
Speed-o'-Sound Sonic was Flashy Flash's classmate in the 44th class during his time as a trainee in the Ninja Village. Their relationship so far is unknown but Flash is responsible for making him the only survivor of the 44th graduation class. During the conversation with Gale Wind and Hellfire Flame, Sonic shows no ill-feeling toward Flashy Flash.

In the webcomic, it is revealed that Flash used to be Sonic's classmate during his years a ninja-in-training back at his hometown. They met during a training session, where Sonic offered Flash tips on how to use bladed weapons. From that point on, they began to train together, with Sonic offering a helping hand without asking anything in return. Both ninjas enjoyed the company, with Flash noting their friendship allowed him to regain some of his emotions lost from the hellish training of the Ninja Village. The two of them shared a passion for justice and desire to change the institution of the Ninja Village. During graduation day, Flash eventually killed all his classmates but spared Sonic's life because of their friendship.

Sonic appears to hold a certain kind of grudge against Flash for mercilessly killing everyone of the Ninja village and not discussing the plan with him. Due to his views on heroes, he believes Flash hasn't changed from his time at the village, since he's doing the same thing he did before, and that even though he changed his title, he didn't change his way of life. Because of their differing ideologies, Sonic no longer treats Flash with the same friendship as before, only working with him against the Heavenly Ninja Party out of necessity and convenience, and putting Flash on his number two spot for heroes he plans to defeat.

Blast
Sonic holds a deep loathing for Blast for his actions against the Ninja Village. Nine years ago, the hero attacked the village to eliminate the villainous group once and for all. During his attack, he encountered the ninjas-in-training from Class 5, who attacked him together. But because of their inferior ninjitsu, they were all mercilessly slaughtered by the hero. Sonic managed to escape the carnage, although the experience left a deep impact on his psyche and how he views heroes and justice. He developed the ideology that the Hero Association is basically the same as the Ninja Village by killing their enemies but simply doing it for the greater good based on Blast's merciless slaughtering of his comrades.

Hammerhead
Due to Sonic killing all his followers, Hammerhead despises Sonic.

Deep Sea King
Sonic holds a grudge against Sea King for their battle and desires to kill him. Ultimately, Sonic couldn't get his revenge as the Sea King would be killed by Saitama.