Knuckles the Echidna
His name is spelt M-a-r-i-o, and he and Sonic both appeared in Mario and Sonic at the Olympic Games. The rivalry between them is friendly not fierce.
Sonic and Mario's parent companies (SEGA and Nintendo, respectively) has a heated rivalry in the 1990's. It was assumed the two mascots were rivals as well. Nowadays SEGA and Nintendo have a much more friendly relationship however, and Sonic and Mario have even appeared in games together.
Knuckles was introduced in "Sonic and Knuckles" as a rival to Sonic who tries to steal his Chaos Emerald. Their rivalry is still there, but now it's more of a friendly rivalry. BEST FRIEND though, I dunno... Tails has been with him since Sonic the Hedgehog 2, and they're really good friends.
Sonic's main enemy is Dr. Eggman. He also has a rivalry with Jet the Hawk.
A friendly rivalry.
Sonic and Shadow do indeed fight on several occasions. Most notably, they have two rival battles in Sonic Adventure 2, where they were enemies for majority of the game. Now, they tend to lean more towards a friendly rivalry, with their clashes having a reason beyond wanting to outdo each other.
To celebrate homecoming with friendly rivalry
No there friendly rivals.
mobianangel is a person on youtube who does video's such as glitche's with sonic, sonic comix, friendly brawls,ect
He first appeared in issue #1 of "Sonic's Friendly Nemesis Knuckles".
His name's Knuckles he is guardian of the master emerald he's also Sonic's friendly rival/teammate.
the are buddies but they both like sonic so friendly rival like sonic and knuckles