MainFragment.kt

class MainFragment : androidx.fragment.app.Fragment() {

    companion object {
        fun newInstance() = MainFragment()
    }

    private lateinit var viewModel: MainViewModel
    private lateinit var message: TextView

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View {
        val view = inflater.inflate(R.layout.main_fragment, container, false)
        message = view.findViewById(R.id.message)
        return view
    }

    override fun onActivityCreated(savedInstanceState: Bundle?) {
        super.onActivityCreated(savedInstanceState)
        viewModel = ViewModelProviders.of(this).get(MainViewModel::class.java)
        viewModel.pokeCharacteristic.observe(this, Observer { pokeCharacteristicResponse ->
            message.text = pokeCharacteristicResponse.toString()
        })
    }

}

class MainViewModel : ViewModel() {
    val pokeService = RetrofitClient().getRetrofit().create(PokeApi::class.java)
    val pokeCharacteristic = pokeService.getCharacteristic(1)
}