Chef Kawasaki minor side character? Side character maybe, but by no way minor. In Rainbow Curse, he, Dedede and Metaknight are the only characters not involved in the story who got a figure (Drawcia also, but she's partially involved), confirming him as one of the most important side characters, if not the most important. In the anime he has a big role as well; the anime is not canon to the games but it still exists in the franchise, and in that production, Chef Kawasaki is important (Nintendo is releasing an app for watching Nintendo-related anime in streaming on 3ds and Kirby is one of those anime - look at Pikachu, he is the main pokèmon only in the anime, in the games Pikachu is simply a random pokèmon). A character doesn't have to be playable in a game in order to be in smash, look at Zelda, now she is playable but when she was intruduced, she had never been playable outside from an obscure CDi game.
About the side characters, Bowser Jr and the Koopalings, and Sheik already are side characters... even Rosalina is a side character, except for her being unlockable as playable in 3D World; and she's in.
Honestly, Magolor feels a lot more like a minor side character than Kawasaki, since almost each new Kirby game has a new final boss and a new set of one shot characters (after KRTD Magolor only made some very minor appearances, mostly in minigames). Dark Matter, Marx and Drawcia are on the same level of him, and all those bosses make lots of cameos as well.
As for being an interesting character, gameplay wise, 90% of Kirby's cast is interesting, due to the creativity of the franchise itself (this is the main reason for why I support other characters over Bandana Dee, because in the Kirby franchise there are so many interesting gimmicks that I'd like to see before having a simple spear fighter; and the main reason for why I support Adeleine, because the paint concept is very recurring in Kirby and I think that she's the most representative character of that type that's not a random boss - the other one would be Drawcia but I simply can't se her in Smash).
Anyway, Sakurai choses characters with marketing and personal tastes in mind; he puts characters from recent games with higher priority; the importance of a character to the series means nothing to him (otherwise Ganon would have been playable over Ganondorf time ago).
EDIT: Also, I think that Sakurai is not alone in the choice of the characters; probably some of the characters are even imposed by Nintendo.
(I like his design, personality and he has potential for an interesting moveset...also more Kirby!).
