Speaking of Sonic characters, I stand by my belief that
Sonic,
Tails,
Knuckles,
Amy,
Shadow,
Rouge,
Cream and
Blaze are the "optimal" selection of characters (within the confines of the Modern Era).
You've got four base Playstyles (Speed, Flight, Power and "Hammer"), none of which stray so far away from the core Sonic gameplay style as to be alienating (like, say, Big the Cat, the Adventure Gunner levels or Silver's playstyle). The selection is weighted towards the Speed style (Sonic, Shadow and Blaze) while limiting the slowest, "hard mode" style to a single character (Amy).
Each character within the styles has their own unique quirks to differentiate them from the other(s) in that subset.
Speed: Shadow has his Chaos Spear, while Blaze has her Fire Magic
Flight: Cream has Cheese for projectile attacks
Power: Rouge can also fly, giving her greater aerial mobility than Knuckles.
Level design can get fairly complicated if you build them with the characters capabilities in mind, examples include: over-hanging ledges that only Tails, Cream and Rouge can get to; ring trails only the Speed characters can use; doors that only Knuckles and Shadow can open because they require the use of Chaos Power; Buttons that only characters with a drop attack (Sonic, Knuckles and Rouge) or the Piko Piko Hammer (Amy) can use; Small gaps only Cheese can get through to open routes.
...I've put too much thought into this, haven't I?