Calling Sonic & Tails "friendly rivals" is very inauthentic. I see where that Tails anon is coming from, but as a Tails self inserter myself, I have to say that framing it as a "friendly rival" is only half-true.

Sonic is the coolest guy ever, and Tails is always trying to catch up and prove himself. From the very first time they met, to the Adventure games where Tails' stages are literally racing Sonic... Tails sees Sonic as someone he has to measure himself up to. SA1 is "Racing" for Tails' stages because that is exactly what Tails' perspective IS. Tails is racing against Sonic, Tails is pushing himself to beat Sonic in a race.

A one-sided race, that Sonic is not aware of. Sonic isn't paying Tails much mind, Sonic is just focused on getting to the goal and keeps an eye on Tails along the way. Sonic is impressed with Tails' ability to keep up, and encourages him to catch up and stuff, but it's not a true race or rivalry on Sonic's end outside of very specific moments.

Alongside that, Tails' motivation is not to "beat Sonic". Tails' motive is to be like Sonic, and become someone Sonic can rely on. Tails' role as a sidekick comes with the necessity of proving that he can keep up with and prove useful to Sonic. Why would Sonic even need a sidekick? What can Tails actually do to help Sonic, of all people? Tails has to answer that question, through his actions. Prove himself as "Player 1.5" or "Player 2".

The Friendly Rival character is Knuckles, who stands in opposition to Sonic even as an ally. Knuckles criticizes and challenges Sonic in ways that Tails fundamentally doesn't. There is a fundamental difference between Tails and Knuckles' dynamics with Sonic. Tails competing with Sonic, and Knuckles assisting Sonic, are very different from what the other typically does. When Tails competes with Sonic, it's to prove his own worth as someone Sonic can rely on. When Knuckles assists Sonic, it's putting his trust in someone he doesn't typically agree with.