The line "No robot I've built..." (as opposed to "No robot I'd build...") states he's got a history of machinery, though we have yet to see it. Good for him though, this means the Sonic cast can now be just a little bit more diverse with it's characters.
Knuckles the Echidna's father dies of pancreatic cancer in the Archie Comics.
"What about Uncle Chuck?" According to Ian Flynn, they kinda just got away with this blue hedgehog who looks like Sonic with facial hair whom Sonic calls "Uncle" as someone a lot of people just call that. See look, "Uncle to many", seems legit.
In the Sonic & All-Stars: Transformed tie-in comic (Sonic Universe #45), Danica Patrick hears threats of interdimensional terrorism by Eggman, yet only takes offense when her racing career is on the line.
In the Archie comics, the song "E.G.G.M.A.N.", from Sonic Adventure 2 appears to exist in-universe, as Eggman sings it to himself in Sonic Universe #39. Whether or not the song exists in it's entirety is unknown.
Amy Rose seems to be the only character in the main Sonic the Hedgehog cast who can handle a casual punch from Knuckles.
Scott Pilgrim's high school band "Sonic & Knuckles" is a very subtle reference to the 1994 Sega Genesis game Sonic & Knuckles.
Sonic the Hedgehog does not know what hedgehogs look like.
Whisper the Wolf's design began with an in-game custom character in Sonic Forces. From there, she would gradually evolve into her final look.
Tails is very discontent with the fact that Eggman has settled on making Tails Doll rather than a Metal Tails. Moreover, this suggests that he finds Metal Sonic and Knuckles to be particularly cool.
When robots or lackeys fail, Eggman uses his built-in command grab
In 2015, it was revealed that Ian Flynn, Tracey Yardley!, and Ben Bates of Archie Sonic the Hedgehog fame had pitched a line of Super Mario comics to Nintendo. It was ultimately rejected. Allegedly, Mario was also intended to cross over with Sonic and Mega Man in comic form.
Knuckles the Echidna's father dies of pancreatic cancer in the Archie Comics.