1) Implement getter with logic

Why not just this?

data class User(val firstName: String, val lastName: String) {
    val fullName = "$firstName $lastName"
}

I'm going to be that annoying I18N guy: some cultures put the last name first. For that reason too,

givenName

and

surname

are better property names.

haha, that's only an example, but thanks @Alan Evans Valid point!