I want to know the appropriate word for antonym/opposite for 'Kafir'
مؤمن (Mu`min) is the antonym for (Kaafir) كافر. it means believer, whereas kaafir means non-believer.
kaafir has another meaning which is ungrateful. In such a case, the antonym would be شاكر (shakir) - which means grateful.
Whereas Urdu could be considered as a language which has many similarities to Arabic, especially in Fiqh, as a result, it could be declared that:
The word believer or (Mo'men)مومن can be selected as the most adequate equivalent for the antonym of Kafir.
It can be surveyed from this another aspect that: whereas the word Kafir almost means unbeliever in English, then its antonym could be termed as the word "Believer" which means: مومن (based on my common knowledge).