Enum classes as any other classes can have a constructor and be initialized

enum class Color(val rgb: Int) {
    RED(0xFF0000),
    GREEN(0x00FF00),
    BLUE(0x0000FF)
}

Enum classes can also declare members (i.e. properties and functions). A semicolon (;) must be placed between the last enum object and the first member declaration.

If a member is abstract, the enum objects must implement it.

enum class Color {
    RED {
        override val rgb: Int = 0xFF0000
    },
    GREEN {
        override val rgb: Int = 0x00FF00
    },
    BLUE {
        override val rgb: Int = 0x0000FF
    }

    ;

    abstract val rgb: Int

    fun colorString() = "#%06X".format(0xFFFFFF and rgb)
}

enum class Color {
  RED, GREEN, BLUE
}

Each enum constant is an object. Enum constants are separated with commas.

Enums can be mutable, this is another way to obtain a singleton behavior:

enum class Planet(var population: Int = 0) {
    EARTH(7 * 100000000),
    MARS();

    override fun toString() = "$name[population=$population]"
}

 println(Planet.MARS) // MARS[population=0]
 Planet.MARS.population = 3
 println(Planet.MARS) // MARS[population=3]

Remarks

Just like in Java, enum classes in Kotlin have synthetic methods allowing to list the defined enum constants and to get an enum constant by its name. The signatures of these methods are as follows (assuming the name of the enum class is EnumClass):

EnumClass.valueOf(value: String): EnumClass
EnumClass.values(): Array<EnumClass>

The valueOf() method throws an IllegalArgumentException if the specified name does not match any of the enum constants defined in the class.

Every enum constant has properties to obtain its name and position in the enum class declaration:

val name: String
val ordinal: Int

The enum constants also implement the Comparable interface, with the natural order being the order in which they are defined in the enum class.