Do data classes need unit tests if we know they only have a primary constructor and no methods? data class(val id: String, val firstName: String, val lastName: String)

That depends entirely on your testing strategy.

It is like

assertEquals(4, 2 + 2)

— sometimes it does make sense, but usually it does not.

I would say that data classes are already covered at many places when testing is concerned

Not for unit tests. The behavior of the data class is already tested by JB at language/compiler level. Can be tested as part of integration tests