adumbrodeus
Smash Legend
No weakness to sonic...I'd say it's Pit.
Difficult learning curve I'm not sure about... you can basically spam most of his moves. But he can stop campy characters with the shield. Spams all over slow characters with his Angel Ring and a few other moves, but runs fairly slow and his up+b has some slow startup time. Some of his weaknesses preventing him from high tier would include his gimpable recovery and the fact that he can get caught in some of his spammable moves, allowing for a quick punish from the enemy.
Seriously, the hint was very specific about not being able to do anything to fast characters, so if the char isn't good against sonic, that tells us right there that it can't be that character because Sonic is pretty much the poorest-performing of the really fast characters, so he'll have the fewest match-ups where he has the advantage, and any char with a disadvantage against him is therefore a good possibility.
Actually, I had a thought...
Captain falcon is a possibility, spacing is absolutely essential for him because of his overall weak game, just so he can survive. He can definitely spam all over slow characters with his speedy attack and retreat tactics, but chars who have about his speed can simply keep up with him far too well for him to use these tactics so he just gets obliterated. He's also definitely underplayed.
Regardless his massive list of disadvantages means in spite of the fact he can spam all over slow characters, he's generally not successful in actually winning versus them. He's got a couple of glaring weaknesses as well (being built to combo in a game where low hitstun prevents that) and theres no way he'll ever... EVER get into the higher tiers.
Except in Items play with the timer as the only item...