Just for the hell of it, here's a list of all the secrets that I can remember off the top of my head. This isn't every single secret/easter egg, just the ones that directly affect gameplay.
Name Easter Eggs:
At the beginning of the game, if you name your character any of these things it'll have different effects
- "Link" - Guts+2
- "Zelda" - Will+2
- "Ganon" - Str+2
- "Richter" or "Alucard" - Changes a line of dialogue in Jonas's intro
- "Chief" or "John" - Changes a line of dialogue in Ataraxios's intro
- "Reimu", "Marisa", "Cirno" - Makes it easier to find Rykah's fight
Secret Scenes:
Poco - In the hotel, after you destroy the generator back track to the room full of wires and a single box containing an energy drink (I think) In the bottom right corner, there's a unique small trash can object you can interact with. It'll transport you to the tiny raccoon house, where you can get fucked by Poco.
Alex - In the first room of the Convention Hall, there's a door that requires a keycode to access. Input either 5010 or 9053 and it'll unlock, where you can fuck Alex. (There's a few other possible codes, but I don't remember them off the top of my head.) There's also an energy drink in the room he's standing in, a callback to Tailbound: Classic.
Both of their dialogue changes depending on which one you meet first.
Secret Boss:
There are a few methods of reaching the secret boss, so I'll list them all in order. Generally though, fighting Rykah requires a new game (or newgame +) since she's only in the tutorial area of the game.
- After receiving the sword & Gyronet hoodie, play through the tutorial like normal. Make sure to kill every enemy along the way, and do not get hurt. (If you fail either of these, it won't unlock Rykah.) Once you get to the first save kiosk, save and backtrack to the room where you got the sword. A stairway will be revealed, with Rykah waiting below. (The stairway is in the upper left corner of the room next to some boxes, right below the doorway leading out.)
- Start a new game and name your character either Reimu, Marisa, or Cirno. After getting the sword, simply exit and re-enter the room, and the staircase will appear.
- With a Newgame+ file, all you have to do is enter and re-enter the room after getting the sword. However, Rykah's fight (like all other boss fights in NG+) is faster and she has a larger health pool.