With a roster as massive as that seen in Super Smash Bros., making sure each fighter is balanced can be a tricky business - hence the issues with Diddy Kong, which have recently been patched.
However, there are some Smash fans who feel that creator Masahiro Sakurai has deliberately tinkered with the power of certain characters - namely the ones operating in franchises he has had some personal involvement with - to make them more powerful than the rest of the cast.
Everyone loves a good conspiracy theory, but Sakurai has shot this one down in flames in his regular column in Famitsu:
With Smash Bros., every now and then someone will vent their misplaced anger towards me. They say that I 'prioritize the strength and playability' of characters that I've created.
First off, what merit is there for me to play favorites with characters I worked on? To bring up an example, two characters I have trouble with and can't use very well are Pit and Palutena. Therefore, for me, I feel that 'Pit and Palutena are weaker than the average character.' If I were to adjust them to fit myself, they would become even stronger – but as you may notice, I haven't.
Sakurai goes on to explain that while he is responsible for deciding the ultimate balance of the game, he has people helping him monitor things to ensure everything remains perfect. Naturally, some things do fall through the cracks - hence the aforementioned Diddy Kong fix - but that's to be expected with so many characters on offer.