The Sonic series is never in a rush to make many callbacks to the infamous Sonic '06. This is understandable, given that the game was universally poorly received and largely seen as the worst Sonic game ever released.
However, the Knuckles TV series just bucked this trend, throwing in a reference to the antagonist, Iblis. The character design is clearly taken from his appearance in Sonic '06 too, making this a pretty deep cut that won't mean much to most viewers.
This was spotted by Twitter user AlenWG, who has compiled a huge thread on all of the different references and easter eggs in the Knuckles show. To make a long story short, there's a lot of them, with even lesser-known entries getting a nod.
In Sonic '06's case, it's obviously very infamous in the fanbase, but more casual viewers probably aren't aware of the absolute stink it left on the franchise. Even younger fans of the games wouldn't have been around for its launch, so they might now know what all of the fuss was about. In fact, according to the Sonic Wiki, this is actually Iblis' first appearance since being mentioned in Super Smash Bros. Brawl in 2008.
Sonic '06's actual title is just Sonic the Hedgehog. Since that would make things very confusing, it is almost exclusively referred to as Sonic '06 instead, since it launched in 2006 to coincide with the series' 15th anniversary.
Unfortunately, outside of a few neat references being spotted by fans, the conversation about the Knuckles show online is far from positive. As soon as it dropped in North America yesterday, fans were immediately disappointed, particularly with its focus on the human characters instead of the titular echidna. In other words, Knuckles doesn't have enough Knuckles.
At the very least, fans have the next film to look forward to this winter, with Sonic the Hedgehog 3 set to hit theatres on December 20. It's said to "take a lot" from the fan favourite Sonic Adventure 2, which includes its main theme song, Live and Learn
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