Red Key (Wario: Master of Disguise)
From the Super Mario Wiki, the Mario encyclopedia
The Red Key is a key found in Wario: Master of Disguise. It is one of three colored keys found in Allergia Gardens, the other two being the Yellow Key and Blue Key. The purple treasure chest containing it is in the top-left corner of a room with red mushrooms that spawn moving platforms when Wario stands on them. The chest is reached by either pressing the mushrooms in the correct order or using Arty Wario's blocks. The Red Key opens two locked doors of the same color.
After Wario obtains the key, the number 2 appears next to the key's icon on the top screen. After unlocking one of the doors, the number changes to a 1, implying that the Red Key is actually two keys. However, when obtained, Wario refers to the key as a singular entity. It shares this trait with the Yellow Key and Master Key.