But Krystal doesn't even have any development, or is interesting in the slightest.
In Adventures, you play her in the tutorial, the rest of the game she's trapped as typical damsel in distress, and the only reason why Fox bothers saving Sauria is cause he got a look up her skirt. In Assault, she... uh... well... doesn't really do anything. She's a member of Fox's team, and a rather incompetent one at that (I mostly remember having to rescue her in that game). And in Command, she flipflops between Star Fox and Star Wolf and is overall pretty insignificant to the plot, at least from what I remember (I don't remember much).
Calling Star Fox characters, who have about as much character development and personality as a common rock to be "more developed and interesting" than anything is pretty insulting. And I say that as someone who really enjoys most Star Fox games, and like most of the characters.
Ridley, on the other hand, has been Samus' arch enemy since the very first game, has been in every Metroid except for like 2 games, is considered an iconic character of the franchise, has a lot of personality and backstory going on, especially with all the different forms he takes during the plots of these games. While he doesn't talk in the games, he has been central to any Metroid story he has been involved in.
To top it off, Metroid is one of the biggest franchises Nintendo has, and it still has only 1.5 characters, whereas Star Fox, a series that has not seen a new game for 8 years (beyond a 3DS remake), has twice as many already.
Finally, I don't believe that, if Krystal EVER were included in Smash, that she should use the staff. It would not be representative of her because she held the staff for a total of 5 minutes during the tutorial of Adventure. In Assault and Command, she did not use the staff, it wasn't even shown to exist anymore at that point. If anyone should use the staff from Adventure, it would be Fox, and that's not gonna happen either.