This is a tough one. If we're talking generally, it's easier for me to think of users than canon so... Cerin would be my rival, though it's a friendly positive rivalry, not a bad one. Other allies would just be other friends and the like. My boyfriend Phyre would be a great ally, as would Muki and Dewwy among many others.

I could probably speak better from a story perspective in regards on Mobian Leaf in a Sonic AU world, since I kinda have that canon going on, in which I accidentally tie him up with Shadow to a degree which at best can make them allies, or at worst affiliated. Still working that part of the story out. In more general/relative terms, I guess Leaf would rival Knuckles as a fighter, I tend to make Leaf pretty strong but not fast that he might be able to give Knuckles a run for his money in a fight, but while I had considered them crossing paths, there was never a necessary need for them to fight.

Story wise, the only canon rival I had was technically in a negative sense with Infinite, as I have a story idea in my head which runs parallel to Sonic Forces but as a B side of what things we don't see from either Sonic's perspectives or that or the Avatar. No major heroics until the very end, but while Infinite deals with Sonic independently, he has run-ins with Leaf and his crew Team Spectre multiple times and ends up cursing him with a virtual power that he suffers with for a couple years. In a post-Forces world, Leaf generally stays away from the resistance due to a grudge with them blowing up the Death Egg which still housed many captured mobians, those that Leaf and crew managed to save prior to the orb's detonation. I dunno about if there's any canon allies he becomes comfortable with worth mentioning, as the AU kinda focuses more on my own characters I made for that world, along with a city built on the ashes of old.

I still need to iron all that out, so if you want to know more, feel free to ask, but know while I have the ideas, I don't have anything written down so it's very spotty what I'm trying to bring together.