YellowKey
The YellowKey (JP: イエローキー) is a Key Item in Chapter 5 of Deltarune.
The YellowKey is acquired in Flower Castle after defeating Yellow's Bullet Pattern and convincing him that he trampled Blue's flowers. It is used to open the Yellow Door in the west wing of the castle. The room inside is revealed to be the start of "THE WAY THAT PUNISHES YOU (BAD WAY)", prompting Susie and Ralsei to find Yellow.