Miyu and Fay, in case you are unaware, are the two brand new recruits that joined the Star Fox team in the cancelled follow-up to Star Fox (which was eventually released via the Super NES Classic Edition in 2017). They resemble a lynx and a poodle respectively, and have since become popular characters among members of the fanbase despite having only ever appeared in a single game.
The new hack, therefore, seeks to give players more time with the Star Fox recruits, incorporating them into the original game with fresh dialogue being added and existing cutscenes being edited to make this as natural as possible.
You can grab the patch now from the ROM Hack Plaza, should you want to check it out yourself.