Found this out today. You can play as Krystal in the single player mode of Star Fox Assault. It just requires an Action Replay code.

Force Player 1 to be Krystal USA: 002A2CA3 00000003
Force Player 1 to be Krystal PAL: 002BD063 00000003

This only works on levels where the Krystal model is loaded, which is level 6 (Sauria) and level 9 (Aparoid Homeworld). On other levels where Krystal never accompanies you out of the Arwing, your character shows up invisible and most functions don't work. If you do certain moves, your controls no longer work, soft-locking the game.

You can even encounter the Krystal who joins you on the mission. It's like looking in a mirror!

It is suspected that with some additional codes, Krystal's model could be forced to load into all areas, similar to how the Krystal Mod for Adventures, "Amethyst Edition" works. If so, Krystal could probably become playable in all on-foot missions. A few more codes, and we could replace the Arwing Fox model with the Arwing Krystal model. I wonder if the creator of this code, ModSault, will look into this further. You can check out the original comment with these codes, codes to do the same thing for Slippy, and links to a ton of other codes for Star Fox Assault on this Reddit thread.

Thanks to Niko on the Discord server for the heads up.