Sonic's eternal arch-enemy and nemesis, whom he has known longer than anyone else (mostly since Sonic saw Robotnik first than Tails). Along with Tails, Robotnik is one of the figures who has often shaped Sonic's destiny. Because of Sonic's hatred of anything unjust, he holds a great mutual loathing for the doctor and his evil plans to take over the world and has spent years rescuing the victims of Robotnik's conniving schemes. Whenever Robotnik hatches a plan to conquer the world, Sonic always shows up and thwarts him. Because they always fight each other to win, Robotnik has become a longtime rival of Sonic's as well as an enemy. Also, whenever Sonic's anger explodes over injustice, he always makes Robotnik the receiving end of this aggression.

Despite his dislike of Robotnik, Sonic has never really taken account of his adversary and does not consider him his enemy in the true sense of the word. Whenever dealing with Robotnik, Sonic remains fully confident that he can spoil his plans and never really takes them seriously. In fact, Sonic finds entertainment in spoiling Robotnik's plans, seeing as a way of making his life an adventure, and is so used to beating him that he considers it an everyday job.

Miles "Tails" Prower is Sonic's best friend and sidekick, as well as his most faithful and loyal friend. Besides Robotnik, Tails is one of the figures in Sonic's life who has often shaped his destiny. Sonic first met Tails on South Island where he noticed Tails following him. Though he was not interested in Tails at first, Sonic grew impressed with his tenacity when he kept up with him and let him tag along. When Dr. Robotnik then attacked South Island, Sonic joined Tails when the fox followed him into the conflict. In time, Tails proved himself a great ally to Sonic, and after defeating Robotnik, the two had built the foundation for the friendship that would define their future. Eventually, Sonic took Tails under his wing as his sidekick and took upon himself to train him. Since meeting each other, Sonic and Tails have been inseparable, their friendship having grown to the point where they have a brotherly relationship. Mighty even noted they have an "Unbreakable Bond". Sonic himself sees Tails as a cute "adopted" kid brother and looks over him accordingly. Nevertheless, he respects and trusts Tails immensely and treats him as his equal partner in their adventures. Whenever on an adventure, Sonic always asks him to come along and will accept his help at any time. That said, however, Sonic will at times ignore Tails when he gives advice or information, often when it is most useful, though he is big enough to admit he should have listened to him. When Tails finds himself in doubt, Sonic is always there to give him moral support. Throughout their adventures, the two have proven themselves a perfect team who can handle nearly anything; both provide strengths to the other's weaknesses (Sonic handles the muscle while Tails handles the smarts) and neither of them would get far without the other's aid. Tails as in particular proven himself Sonic's most reliable ally, always providing him with invaluable help, knowledge, and machines. In turn, Sonic is impressed with Tails' work and considers his technical skills unparalleled, though this faith makes him leave all the tech work to him.

Sonic's greatest rival and best friends. They have known each other for a long time since Sonic 2, but their relationship is somewhat complicated, the two of them having been summed up once as "best frenemies." When Sonic first met Mighty in Sonic 2, even when Sonic rescued him, they started off as enemies, with Mighty thinking he was Metal Sonic and kept a eye on Sonic. However, when the armadillo discovered that Dr. Robotnik had tricked him and Metal was completely different, the two of them joined forces to stop Robotnik, and they departed on good terms, befriending each other.

Sonic and Mighty are much like oil and water. While Sonic exemplifies the wind and is laid-back, cool and free, Mighty exemplifies the mountain and is stern, serious and unmovable, which cause them to argue a lot. They share a competitive rivalry and often end up fighting when clashing. With Sonic's speed equaling Mighty's strength in every way, their fights are always climactic.

Due to his lack of social abilities, Mighty seems to fall for Robotnik's deceiving very often, which, in turn, would be the cause of most of his confrontations with Sonic (and to a minor extent, Tails), although as time passed, the armadillo has generally become more aware of Robotnik's plans and has stopped believing his claims regarding the hedgehog.

Despite their differences, Mighty is considered Sonic's best friend next to Tails, and is a vital part of the team. The two of them are connected by a hot-blooded friendship and according to Tails, they are "funny" together.

Compare to Mighty, Sonic and Ray have a more friendly relationship. With Ray being more open to others compared to Mighty, he openly accept Sonic as friend when he saved them and is one of the reasons the Freedom Fighters are a team. Which leads to Sonic rather talking to Ray then Mighty for this reason