Lies of P uses a lot of the tricks that Dark Souls, Bloodborne, and even Elden Ring use. Key items that can prevent the player from progressing in the game by being very well hidden. One of those items is the Rosa Isabelle Street Key, and here is where to find it.
Sometimes those bosses tend to hide elements crucial to the game—in this case, a whole new unexplored section of the game—so we are here to tell you how to get to that new section.
Where to find Rosa Isabelle Street Key
After you have fought and defeated the Eldest of the Black Rabbit Brotherhood boss, you will want to go back to the Hotel Krat and make your way up the stairs toward Geppetto's office.
Once there, you will be prompted to speak to him about the Malum District and the Black Rabbit Brotherhood. Exhaust the conversation and go back down the stairs.
Once you are by the stargazer in the main lobby, go into the library where Antonia is located (the lady in the wheelchair). You will now be prompted to speak to her, and she will have a new conversation regarding Rosa Isabelle Street.
Once you exhaust the conversation, she will give you the key to the door that leads to Rosa Isabelle Street. Once you do that, head back up the stairs and search for a corridor that has a big, dark door at the end of it. Now you will be able to open it with the newly acquired key.
The door leads to a hallway that has another door at the end of it. Going through it will put you at the start of the next level, Rosa Isabelle Street.