Uncle Chuck--The Legend

In early canon, "Uncle Chuck" was a very important character. A good inventor opposed to the malicious Dr. Robotnik, early works credit Chuck with inventing the "power shoes" (speed-up shoes) and "power rings" (rings). 

Of Sonic's relatives, he's the most reoccurring. The details change, but at its simplest, Chuck was close to Sonic and helped inspire him to take on Dr. Robotnik. 

Sat Am

In one of Sat Am's most emotional moments, Sonic reunites with his robotized Uncle Chuck and tries to use a power ring to save him. Eventually (in Season 2) they are able to restore his personality and Chuck is able to help them. However he remained robotsized on the outside. It was also revealed that "Charles" himself created the Robotsizer--to prolong life--but "Julian" Robotnik took the design for his own dark purpose. 

Underground

Uncle Chuck reappeared in Sonic Underground as a part of Sonic's origins. While his exact relation to Sonic/Aleena is not made clear, it's assumed (via appearance) that they are still related. At some point Sonic met Chuck in his early childhood, and after his adoptive family was robotisized (and his home burned to the ground), Chuck took him in. Chuck taught him to be a freedom fighter, but like his Sat Am counterpart he was robotisized. 

"Grandpa Chuck" of Sonic X

Despite relying more on the games and Sonic Team for background on Sonic X, some elements of Western canon leaked into the anime. One of these elements: Grandpa Chuck (Chuck Thordyke). Chuck is Chris' grandfather, rather than Sonic's uncle, and he's a human. However he shares his intelligence and talent for inventing. Over the series Chuck would grow closer to Tails, becoming a role-model with an almost uncle-like relationship to him.  

Comics

Uncle Chuck was faithfully adapted into the Archie Comics as Sir Charles the Hedgehog, brother of Sonic's father, Jules (and brother-in-law to Bernadette). Later in the series run, he was restored to his Mobian self, defended Sonic and the others as a member of a council, and helped with the Secret Freedom Fighters (featuring the likes of Sally's brother, Elias, and Silver the Hedgehog).  

Following the reboot, Sega created the condition that game characters could not have family members unless they already appeared in the games (such as Cream's mother, Vanilla).  Ian Flynn got creative and brought Charles back with the label "Uncle to Everyone," under the guise that his relationship to Sonic was adoptive (this character was allowed to return because he came from the Sat Am TV show, rather than Penders-written comic characters like Jules and Bernadette). 

Despite this, Sonic was shown to be very close to his "Uncle" and Charles likewise tried to look out for Sonic. Over the course of the Unleashed-adaptation, Charles was also revealed as a friend of Professor Pickle. His role in this series, though, would be his last.  

Uncle Chuck survived a comic reboot, appeared in 2 cartoons, and inspired another character. Many have tried to mimic the emotional bond between Sonic and Chuck, but the character remains one of the closest to Sonic, even visually. It's a shame the character's gone for now, but who knows? Perhaps he'll inspire someone else...