Sonic.
Design? I'm fine with that.
Everything that happens once he opens his mouth and words start coming out? No, stop it.
He always just comes across as one of those irritating "surfer dude" stereotypes from the 90's that should have stayed in the 90's. I'm absolutely mindblown that I don't recall him saying "bodacious" or "radical" because that's just how he comes across (he probably says "rad" at some point though"). Add to that him coming across as loud, cocky and arrogant. I really don't find any part of his personality even remotely appealing.
So it's lucky he doesn't actually speak in the games I like.
It might be a fault of Nintendo, but that's still part of the character. You could pass off most disliked characteristics for characters as a fault of the creator, but they're still flaws of the character.
Odyssey being mentioned, I just finished beating the game again this afternoon thus have been recently reminded that this is probably the worst game in terms of Peach looking like she did it all for attention. Like, at the end she just tells Mario and Bowser "no" on their advances and Bowser just...Accepts it? Did she try saying "no" when he initially kidnapped her? If she did, why did he kidnap her the first time but just accept it like a bro the second time? She's been screaming "help me Mario" throughout the game and is then just like "nawh, go away" once he saves her?
There's something to be said about this being a 'hot take' making her less of a prize for Mario to win at the end, but it also comes across very attention seeking.
And yes, Cappy is also very annoying.