In the games, Sonic X and Sonic Boom,
Amy has a crush on Sonic but unlike the official games, she keeps it a secret. It is implied that Sonic reciprocates her feelings, but feel awkward about acknowledging them. There are scenes where SonAmy really shines though, like in one episode, U.T. revealed that Sonic cares for Amy's well being.
In the games, Amy is depicted as driven and competitive. She spends much of her time following Sonic to get his attention or make sure he is safe while demonstrating her affection. Series co-creator Yuji Naka said that Amy was designed "to always chase Sonic", and has made it her life goal to one day marry him.
Sonic originally had a girlfriend
Amy Rose, the pink hedgehog, is known to many as Sonic's love interest. Having said that, she's nearly always seen chasing him and he never reciprocates.
Queen Sally Acorn (Light Mobius) was the Light Mobius counterpart to Sally Acorn, the queen of her zone's Mobius and the wife of King Sonic.
After years has been past, Sally is Sonic's old girlfriend back from Knothole.
Julie-Su was a Mobian Echidna and the girlfriend and distant relative of Knuckles the Echidna. Born the daughter of Luger and Mari-Su, Julie-Su was brought up amongst the Dark Legion.
Sonia (also a speaking voice by Jaleel White and singing voice provided by Stevie Vallance) is the only female hedgehog member of the Sonic Underground and the sister of Sonic and Manic. Unlike her brothers, she was raised by an aristocratic foster family, giving her an upper class mentality.
Tiara Boobowski from the cancelled Sonic X-treme makes a cameo standing next to Amy Rose when Sonic and Sally break up.
In the video games since Sonic Adventure 2, Knuckles appears to have a slight crush on Rouge the Bat that he tries not to show, and Rouge is confirmed to have a crush on him.
Jules Hedgehog was the husband of Bernadette Hedgehog, brother of renowned scientist Sir Charles Hedgehog and father of world-famous Freedom Fighter and hero Sonic the Hedgehog.
Sometimes he appears to like her. But consistently he generally views her as a friend and isn't into relationships. Sega actually has mandates that Sonic and Amy can never be in a relationship so we may never know.
In the game, the very human Princess Elise the Third used the Chaos Emeralds to save Sonic's life and kissed the anthropomorphic hedgehog as she did. Though he didn't work on that particular entry, Iizuka said "I don't think we'll be doing that again".
In “Tails' Crush”, Tails started to have a crush on Zooey during one of her daily routines. They had several encounters, during each of which Tails acted to act cool, romantic, manly or ritualistic towards her in awkward ways, when he only wanted to talk to her.
In the "Mobius: X Years Later" arc, set in an alternate future, Sonic became King of Mobius, married to his love interest Sally Acorn, and had two children named Sonia and Manik (sharing their names with Sonic's siblings in Sonic Underground).
His Dad!? After his father was turned evil because of an experiment gone wrong, Sonic and Robotnik teamed up to kill Sonic's dad with a giant robot. Sonic the Hedgehog and Dr. Ivo “Eggman” Robotnik have been deadly enemies since they debuted in the first Sonic the Hedgehog game in 1991.
Scourge the Hedgehog, Sonic's evil twin, is on the attack! Leading the Antiverse in a full-scale invasion, Scourge's goal is none other than multiversal domination-with Sonic by his side?
Manic The Hedgehog, a character from Sonic Underground, is Sonic's brother and the drummer in the sibling's band. Manic is the most laid back of the family (yes, there's somebody more laid back than Sonic), though perhaps this is because he doesn't have any powers. This never held him back, though.
Amy Rose is shadow's girlfriend!
In his personal life, Knuckles also became the father of Lara-Su through his girlfriend Julie-Su.