Toml

@Serializable
data class User(
    val name: String,
    val account: Account? // Nullability
)

@Serializable
data class Account(
    val username: String,
    val password: String
)

// Now with [Default] instance
val user = User("Peanuuutz", Account("Peanuuutz", "123456"))
// Either is OK, but to explicitly pass a serializer is faster
val tomlString = Toml.encodeToString(User.serializer(), user)
val tomlString = Toml.encodeToString<User>(user)
// Print it
println(tomlString)
/*
name = "Peanuuutz"

[account]
username = "Peanuuutz"
password = "123456"
*/

// And to reverse...
val user = Toml.decodeFromString(User.serializer(), tomlString)
// Print it
println(user)
// User(name=Peanuuutz,account=Account(username=Peanuuutz,password=123456))

// Or you're lazy to create model class, try [TomlElement]
val config = Toml.parseToTomlTable(tomlString)
// Now access to all entry (think you need getByPath)
val password: TomlLiteral = config["account", "password"]!!.toTomlLiteral()