To get the red outfit you'll need to take down the shinbakufu first through all main assassination missions, then wrap up the personal stories for naoe, yasuke and junjiro during the epilogue. Things kick into gear with "out of the shadows" - a peaceful mission at the hideout where you chat with your allies. Just finish that up and watch the credits roll, the legendary master assassin set appears automatically in your inventory after.

The armor itself is pretty special, letting you remove two extra health segments during assassinations while the hood does the same for ground attacks. Worth mentioning you can customize the look at the forge once you have it. Quick heads up though - there's no way to get this early, you've got to complete everything in the main story first, that's just how it works.