Well, that's what she is, though.
However, since she's playable in SMB2, the major character in Super Princess Peach and in a whole lot of spin-offs, whereas she also rules over the place the Mario games take place mostly. That makes her pretty major, imo.
Krystal on the other hand has NOTHING speaking for her except the fact she has, well, boobs (and her only power, that telepathy stuff, is absolutely out of place, imo).
Also, saying that Krystal is Fox' girlfriend is wrong, cause, if I may remind you, she left Star Fox and joined Star Wolf to bang with Panther.
I, as one of my predecessors said, think that Star Fox Adventures never really existed as more than a spin-off or something, because it, and with it Krystal, was never supposed to be a Star Fox game in the first place. This, and the fact that, although she appeared and showed herself more than any member of Star Fox or Star Wolf next to Fox himself, is still the dullest and most unnecessary character in the whole cast, makes me hate her with such a passion.
Unless she shows that she's actually useful I won't accept her into the Star Fox cast and even less into the Smash Bros. roster.
Though, I agree, Star Fox doesn't need another rep.