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, Genos despises Speed-o'-Sound Sonic. 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 audio book, Sonic is chased by Puri at a festival after the latter mistakes a challenge for an offer for sex.

Flashy Flash
Flash used to be Sonic's classmate during his years a ninja-in-training back at his hometown. During graduation day, Flash eventually killed all his classmates but spared Sonic's life. Sonic never seems to hold any grudge toward Flash.

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 wouldn't get his revenge as Sea King would be killed by Saitama.