Deltarune Guide | How to Find Secret Boss and Use Mystery Key in Chapter 5

The Chapter 5 secret boss route starts with the Mystery Key. Before the key becomes useful, you need 10 Pink Coins. These coins are hidden across the Garden, Cliffs, and Castle in pink chests and Pink Bells, usually behind optional platforming, statue puzzles, or backtracking paths.

Once you have 10 Pink Coins, fast travel to the Cliffs and enter the pink shop. The Mystery Key is sold there as a special item. Buy it, then leave the shop; the lock it opens is not near the vendor. The key is only used much later, near the top of the Castle route.

Find the Pink Door

To find the door, progress deep into Chapter 5 until you are near Green’s Shop and the late Castle route. From Green’s Shop, head right, activate the perspective switch, and climb upward through the platforming section. When you reach the upper path, go left instead of continuing toward the main objective. Ralsei warns that the fountain is not that way, which is the correct sign for the optional route.

Keep moving left through the flower-target platforming. When you reach the place where you can drop down, take the pit. Below, activate the green pinwheel and descend through the obstacle section. Cut through the blue flowers when needed and keep your Soul clear of the hazards until you land at the bottom area.

Use the Key

The large Pink Door waits at the end of this path. Return to normal form if you are still in the shifted perspective, then use the Mystery Key on the locked door. This opens the secret boss route and leads to Pink, also known as Mad Mew Mew. 

Save first, stock healing items, and use the nearby fast-travel point if you need to leave and prepare before starting the fight.