Sonic as a whole is a franchise full of wasted characters. I still think the Adventure era was the best time for modern Sonic, maybe not gameplay, but definitely character wise, having actual character growth and such. Nowadays, most of them just feel just kinda... there.
Like Knuckles, who went from Sonic's rival and guardian of the Master Emerald to just some tagalong guy, who doesn't even seem to care for the emerald anymore. Do the modern games even reference it at all?
Tails went from someone who disarms an enormous bomb and infiltrated a government prison facility alone, to someone who covers in fear from Chaos 0.
Amy went from having a crush on Sonic, but not being afraid to stand up against him, to pretty much an crazy, obsessed stalker... more than usual, that is.
Shadow was brought back due to fan demand and for a while they didn't know what to do with him (seriously, alien DNA, realistic guns and flat out killing Eggman?). thankfully he got better, though sadly more or less taking over Knuckles' role as Sonic's rival.
Silver and Blaze keep coming back, despite being from the future/alternate dimension, which does kinda cheapen their existence when said fact doesn't even matter anymore. They just feel like 2 more random characters who are just there as a result. Shoutout to Marina (if that's her name) who doesn't have that luck, and Eggman Nega for being stuck in spin offs.
Really, that's just the tip of the iceberg. There really needs to be another game which fleshes out the characters and retcons some of the more stupid stuff, like Tails' in Forces.