By necessary, I mean a character that has been within that franchise since the beginning (or near beginning, as in by the second installment) and has continually had important roles within each game in the series, or most of them. Luigi, Bowser, Peach, Zelda, Ganondorf, King Dedede, Meta Knight, Falco, and Wolf all meet that (though Falco seems to be no more important than the other members of Fox's team). Diddy Kong was basically the face of Rare's contribution to the DK series and he has fared far better than any other Rare created Nintendo character since Rare left Nintendo. Diddy Kong made regular appearances in the Mario spinoffs and continues to make regular appearances.
By comparison, Dixie Kong's post-Rare appearances have been regulated to Paon's mediocre DK games (King of Swing, Jungle Climber, Bongo Blast) and three Mario sports titles (Baseball, Hoops 3-on-3, and Super Sluggers). K. Rool had the same fate, only with appearing in one Mario spinoff (Super Sluggers).
For this reason, K. Rool and Dixie Kong has basically been absent from the minds of many players since the SNES/N64 days. Keep in mind though that each installment of the DKC trilogy was within the Top 10 highest selling SNES games, so they are still in a lot of people's minds despite their long hiatus.
However, as I said, its perfectly plausible Sakurai takes a look at Donkey Kong Country Returns and sees the franchise is fine with just DK and Diddy as its playable characters, and looks for other ways to rep the series (I am think Rambi is going to be an Assist Trophy this time, and DK will probably have a stage from Returns as well as possibly a new DK Arcade stage).
Also I know my logic can be flawed when it come to Medusa/Hade, King K Rool and Ridley, because I want to have more villains that are playable because I really enjoying to play the villains. Sometime, I would be so evil and ruin their days and games with villains.
K. Rool is more comic relief than actually evil. Even his Melee trophy refers to him as something of a bumbler. The only truly evil major Nintendo villains I can think of are Ridley and Ganondorf. Bowser and King Dedede are more jerks than evil. Meta Knight I don't think has done anything shady/evil since Superstar (though some Dark Meta Knight was running around causing trouble in the Amazing Mirror). Wolf is just a mercenary who fights for whoever pays him the most. Of course you got guys like Black Shadow who are evil (though we never actually see him doing anything evil outside of the F-Zero anime), and Porky of Mother who could also probably count as evil.